Medical device and method for detecting electrical signal noise

ABSTRACT

A medical device is configured to sense an electrical signal and determine that signal to noise criteria are met based on electrical signal segments stored in response to sensed electrophysiological events. The medical device is configured to determine an increased gain signal segment from one of the stored electrical signal segments in response to determining that the signal to noise criteria are met. The medical device determines a noise metric from the increased gain signal segment. The stored electrical signal segment associated with the increased gain signal segment may be classified as a noise segment in response to the noise metric meeting noise detection criteria.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/976,822 filed Feb. 14, 2020, the entiredisclosure of which is incorporated by reference herein.

TECHNICAL FIELD

The disclosure relates generally to a medical device and method fordetecting electrical signal noise.

BACKGROUND

Medical devices may sense electrophysiological signals from the heart,brain, nerve, muscle or other tissue. Such devices may be implantable,wearable or external devices using implantable and/or surface (skin)electrodes for sensing the electrophysiological signals. In some cases,such devices may be configured to deliver a therapy based on the sensedelectrophysiological signals. For example, implantable or externalcardiac pacemakers, cardioverter defibrillators, cardiac monitors andthe like, sense cardiac electrical signals from a patient's heart. Acardiac pacemaker or cardioverter defibrillator may deliver therapeuticelectrical stimulation to the heart via electrodes carried by one ormore medical electrical leads and/or electrodes on a housing of themedical device. The electrical stimulation may include signals such aspacing pulses or cardioversion or defibrillation shocks. In some cases,a medical device may sense cardiac electrical signals attendant to theintrinsic or pacing-evoked depolarizations of the heart and controldelivery of stimulation signals to the heart based on sensed cardiacelectrical signals. Upon detection of an abnormal rhythm, such asbradycardia, tachycardia or fibrillation, an appropriate electricalstimulation signal or signals may be delivered to restore or maintain amore normal rhythm of the heart. For example, an implantablecardioverter defibrillator (ICD) may deliver pacing pulses to the heartof the patient upon detecting bradycardia or tachycardia or delivercardioversion or defibrillation (CV/DF) shocks to the heart upondetecting tachycardia or fibrillation.

A medical device may sense cardiac electrical signals from a heartchamber and deliver electrical stimulation therapies to the heartchamber using electrodes carried by a transvenous medical electricallead. Cardiac signals sensed within a heart chamber using endocardialelectrodes, for example, generally have a high signal strength andquality for reliably sensing near-field cardiac electrical events, suchas ventricular R-waves sensed from within a ventricle. In some proposedor available ICD systems, an extra-cardiac lead may be coupled to theICD, in which case cardiac signal sensing from outside the heartpresents challenges in accurately sensing cardiac electrical events. Invarious medical devices or medical device systems, implantable,transcutaneous, or cutaneous (skin) electrodes may be positioned forsensing an electrophysiological signal by the medical device, which maybe an implantable, external or wearable medical device. Such devices mayinclude devices configured to monitor an electrophysiological signal fora medical condition or health purposes (including, but not limited tofitness trackers, watches, or other medical or fitness devices).

SUMMARY

In general, the disclosure is directed to a medical device andtechniques for detecting noise in an electrical signal sensed by themedical device. The electrical signal noise may be detected in anelectrophysiological signal such as, but not limited to, a cardiacelectrical signal, nerve signal, brain signal, or muscle signal. Thenoise detection techniques may be used in conjunction with a variety ofpatient monitoring devices and/or therapy delivery devices, includingdevices that monitor a patient heart rate. For example, detection ofnoise in a cardiac electrical signal may be included in heart ratemonitoring and arrhythmia detection methods, such as a tachyarrhythmiadetection algorithm, to avoid false arrhythmia detection in the presenceof cardiac electrical signal noise, such as electromagnetic interference(EMI) or non-cardiac myopotential signals. A device operating accordingto the techniques disclosed herein may determine if signal to noisecriteria are met based on an electrical signal sensed by the device andincrease the gain of an electrical signal segment in response todetermining that the signal to noise criteria are met. The device maydetermine a noise metric from the increased gain signal segment. Anelectrical signal segment associated with the increased gain signalsegment may be classified as noise based on the noise metric. The noisedetection techniques disclosed herein may improve electrophysiologicalsignal monitoring by rejecting or ignoring noise segments and may avoiddelivery of unnecessary therapy (or withholding of a necessary therapy)in medical devices that include therapy delivery capabilities.

In some examples, a medical device as disclosed herein may be configuredto detect ventricular tachyarrhythmia, e.g., ventricular tachycardia(VT) or ventricular fibrillation (VF), based on detecting a ventricularrate that is faster than a tachyarrhythmia detection rate for at least apredetermined number of ventricular cycles. The VT or VF rate may bedetected by sensing R-waves from a cardiac electrical signal,determining ventricular intervals or RR intervals (RRIs) betweenconsecutively sensed R-waves, and counting the number of ventricularintervals that are shorter than VT or VF detection intervals.Non-cardiac noise may be oversensed as ventricular R-waves due tocardiac signal amplitude variability and/or due to episodes ofnon-cardiac noise, such as skeletal muscle myopotentials, e.g., duringpatient activity. Oversensing of non-cardiac noise may cause the medicaldevice to falsely increase the count of VT or VF intervals when anunderlying normal sinus rhythm may be present. A medical deviceoperating according to the techniques disclosed herein may detect noisein cardiac electrical signal segments, which may be occurring during aseries of ventricular intervals that include tachyarrhythmia detectionintervals. The device is configured to improve detection of non-cardiacnoise by determining when signal to noise criteria are met and increasethe gain of a cardiac signal segment being evaluated for noise detectionto reveal low amplitude non-cardiac noise pulses that may be present.When noise is detected in the cardiac electrical signal and anarrhythmia detection criterion is satisfied, such as a threshold numberof tachyarrhythmia intervals, the arrhythmia detection may be withheldto avoid false arrhythmia detection and avoid delivery of unnecessarytherapy.

In one example, the disclosure provides a medical device including asensing circuit, a memory, and a control circuit. The sensing circuit isconfigured to sense at least one electrical signal and senseelectrophysiological events from the at least one electrical signal. Thecontrol circuit is coupled to the sensing circuit and the memory and isconfigured to store an electrical signal segment from the at least oneelectrical signal sensed by the sensing circuit in the memory inresponse to each one of a series of electrophysiological events sensedby the sensing circuit. The control circuit is configured to determinethat signal to noise criteria are met based on the stored electricalsignal segments and determine an increased gain signal segment from oneof the cardiac electrical signal segments in response to determiningthat the signal to noise criteria are met. The control circuit isconfigured to determine a noise metric from the increased gain signalsegment, determine that the noise metric meets noise detection criteria,and classify the stored electrical signal segment associated with theincreased gain signal segment as a noise segment in response to thenoise metric meeting the noise detection criteria. The control circuitmay determine that a tachyarrhythmia detection criterion is met fordetecting a tachyarrhythmia based on the at least one electrical signaland withhold a tachyarrhythmia detection in response to the storedelectrical signal segment being classified as a noise segment.

In another example, the disclosure provides a method that includessensing at least one electrical signal, sensing electrophysiologicalevents from the at least one electrical signal, storing an electricalsignal segment from the at least one electrical signal in response toeach one of a series of sensed electrophysiological events. The methodfurther includes determining that signal to noise criteria are met basedon the stored electrical signal segments, determining an increased gainsignal segment from one of the stored cardiac electrical signal segmentsin response to determining that the signal to noise criteria are met,and determining a noise metric from the increased gain signal segment.The method includes determining that the noise metric meets a noisedetection criteria and classifying the stored cardiac electrical signalsegment associated with the increased gain signal segment as a noisesegment in response to the noise metric meeting the noise detectioncriteria. The method may include determining that a tachyarrhythmiadetection criterion is met for detecting a tachyarrhythmia based on theat least one electrical signal and withholding a tachyarrhythmiadetection in response to the stored electrical signal segment beingclassified as a noise segment.

In another example, the disclosure provides a non-transitorycomputer-readable medium storing a set of instructions which, whenexecuted by a control circuit of a medical device, cause the medicaldevice to sense at least one electrical signal, senseelectrophysiological events from the at least one electrical signal, andstore an electrical signal segment from the at least one electricalsignal in response to each one of a series of sensedelectrophysiological events. The instructions further cause the medicaldevice to determine that signal to noise criteria are met based on thestored electrical signal segments, determine an increased gain signalsegment from one of the stored electrical signal segments in response todetermining that the signal to noise criteria are met, determine a noisemetric from the increased gain signal segment and classify the storedelectrical signal segment associated with the increased gain signalsegment as a noise segment in response to the noise metric meeting noisedetection criteria. The instructions may further cause the medicaldevice to determine that a tachyarrhythmia detection criterion is metfor detecting a tachyarrhythmia based on the at least one electricalsignal and withhold a tachyarrhythmia detection in response to thestored electrical signal segment being classified as a noise segment.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem configured to sense cardiac electrical events and deliver cardiacelectrical stimulation therapies according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with anextra-cardiovascular ICD system in a different implant configurationthan the arrangement shown in FIGS. 1A-1B.

FIG. 3 is a conceptual diagram of an ICD according to one example.

FIG. 4 is a diagram of circuitry that may be included in a sensingcircuit of the ICD of FIG. 3 .

FIG. 5 is a flow chart of a method for detecting non-cardiac noise in acardiac electrical signal by a medical device such as the ICD of FIG. 3according to one example.

FIG. 6 is diagram of a first cardiac electrical signal and a secondcardiac electrical signal illustrating analysis performed by a medicaldevice according to one example for determining if signal to noisecriteria are met for increasing the gain of a differential signal fornoise detection.

FIG. 7 is a flow chart of a method performed by a medical device fordetermining a noise metric and classifying a cardiac electrical signalsegment as a noise segment or non-noise segment according to someexamples.

FIG. 8 is a graphical representation of a cardiac electrical signalsegment and corresponding first order differential signal.

FIG. 9 is a flow chart of a method for determining a gross morphologyamplitude metric of a cardiac electrical signal segment for detecting atachyarrhythmia morphology according to one example.

FIG. 10 is a flow chart of a method for detecting a tachyarrhythmiamorphology in a cardiac electrical signal segment based on a grossmorphology signal width metric according to one example.

FIG. 11 is a flow chart of a method for controlling tachyarrhythmiadetection and therapy delivery by a medical device performing the noisedetections disclosed herein according to one example.

FIG. 12 is a flow chart of an alternative method for controllingtachyarrhythmia detection and therapy by a medical device.

FIG. 13 is a flow chart of a method performed by a medical device fordetecting non-cardiac noise and rejecting a ventricular tachyarrhythmiadetection in response to detecting noise according to another example.

DETAILED DESCRIPTION

In general, this disclosure describes a medical device and techniquesfor detecting noise in an electrical signal sensed by the medicaldevice, such as a cardiac electrical signal. In some examples, themedical device may be configured to sense cardiac electrical events,e.g., atrial P-waves attendant to atrial myocardial depolarizationsand/or ventricular R-waves attendant to ventricular myocardialdepolarizations from the cardiac electrical signal. The medical devicemay determine the heart rate or rhythm and a need for therapy deliverybased on at least the sensed cardiac electrical events. For example,atrial or ventricular tachyarrhythmia may be detected by the medicaldevice based on sensed cardiac electrical signals. In some examples, themedical device may be configured to sense R-waves attendant toventricular depolarizations from a cardiac electrical signal for use incontrolling ventricular pacing and detecting ventriculartachyarrhythmias. A ventricular tachyarrhythmia may be detected inresponse to sensing a threshold number of R-waves occurring at a timeinterval from a preceding R-wave that is less than a tachyarrhythmiadetection interval. Non-cardiac electrical noise present in the cardiacsignal, e.g., electromagnetic interference (EMI) or skeletal musclemyopotential signals, may be oversensed as R-waves, resulting in false,short RRIs being determined as ventricular tachyarrhythmia intervals. Insome instances, variability in the R-wave signal strength due to patientmotion or other factors may result in oversensing of non-cardiac noise,leading to relatively short RRIs being counted toward tachyarrhythmiadetection when the underlying rhythm may actually be a normal sinusrhythm. False tachyarrhythmia detection may lead to a CV/DF shock orother tachyarrhythmia therapy delivered by the medical device, such asanti-tachyarrhythmia pacing (ATP), when a therapy may not be needed.

In some examples, the medical device performing the techniques disclosedherein may be included in an extra-cardiovascular ICD system. As usedherein, the term “extra-cardiovascular” refers to a position outside theblood vessels, heart, and pericardium surrounding the heart of apatient. Implantable electrodes carried by extra-cardiovascular leadsmay be positioned extra-thoracically (outside the ribcage and sternum)or intra-thoracically (beneath the ribcage or sternum) but generally notin intimate contact with myocardial tissue. In other examples,transvenous extra-cardiac leads may carry implantable electrodes thatcan be positioned intravenously but outside the heart, e.g., within theinternal thoracic vein, jugular vein, or other vein, for sensing cardiacelectrical signals. Patient positional changes or patient physicalactivity as well as other factors may lead to variation in the cardiacevent signal amplitudes, e.g., P-wave amplitudes, R-wave amplitudes andT-wave amplitudes, in the signal sensed from an extra-cardiovascular orextra-cardiac location. Furthermore, the presence and amplitude ofskeletal muscle myopotential signals in a cardiac electrical signal maybe highly variable due to varying physical activity and posture of thepatient. Cardiac signals sensed via extra-cardiovascular orextra-cardiac electrodes may be more susceptible to signal amplitudevariability and noise contamination, e.g., due to myopotentials orenvironmental EMI, than cardiac signals sensed using transvenousintracardiac electrodes.

The medical device and techniques disclosed herein provide a method fordetecting noise, such as myopotential noise, by increasing the gain of acardiac electrical signal being analyzed for noise to avoidunderdetection of the signal noise by a noise detection algorithm. Byincreasing the gain of the cardiac electrical signal, non-cardiac noisemay be more reliably detected, enabling false tachyarrhythmia detectiondue to non-cardiac noise oversensing to be rejected. As disclosedherein, the medical device may detect noise from a cardiac electricalsignal based on one or more noise metrics determined from a cardiacelectrical signal segment. When the amplitude of noise pulses in thecardiac electrical signal is low, e.g., less than a threshold, the gainof the cardiac electrical signal is increased to intentionally increasethe amplitude of the noise pulses to improve the likelihood of detectingnon-cardiac noise present in the cardiac electrical signal that may beleading to false cardiac event sensing.

Noise detection techniques are described herein in conjunction with anICD configured to sense cardiac electrical signals using an implantableextra-cardiovascular (or extra-cardiac) medical lead carrying sensingand therapy delivery electrodes. Aspects disclosed herein, however, maybe utilized in conjunction with other cardiac medical devices or systemsand more generally with other medical devices or systems configured tosense electrical signals in which the electrical signal can become noisecorrupted. For example, the noise detection techniques as described inconjunction with the accompanying drawings may be implemented in anyimplantable or external medical device enabled for sensingelectrophysiological signals including brain, nerve, and muscle signals.The noise detection techniques may be used in conjunction with medicaldevices configured to sense cardiac electrical events from cardiacsignals received from a patient's heart via sensing electrodes,including implantable pacemakers, ICDs or cardiac monitors coupled tonon-transvenous, transvenous, pericardial, or epicardial sensingelectrodes; leadless pacemakers, ICDs, cardiac monitors havinghousing-based sensing electrodes; and external or wearable pacemakers,defibrillators, or cardiac monitors coupled to external, surface or skinelectrodes. The noise detection apparatus and techniques disclosedherein may be implemented in a variety of medical devices that useimplantable or external electrodes for sensing electrophysiologicalsignals that may be noise corrupted.

The illustrative examples presented herein involve sensing cardiacelectrical signals for the detection of ventricular tachyarrhythmia. Thedisclosed techniques, however, may be implemented in a medical deviceconfigured to sense atrial and/or ventricular cardiac events fordetecting a variety of cardiac rhythms, such as bradycardia,tachycardia, fibrillation, etc. For example, a cardiac device using thedisclosed noise detection techniques may be configured to sense P-waves,e.g., for detecting (and optionally treating) atrial tachyarrhythmia. Inthis case, the medical device may count PP intervals occurring betweenconsecutively sensed atrial P-waves which are less than an atrialtachyarrhythmia detection interval. Cardiac electrical signals, whichmay be sensed from within or outside an atrial chamber, may be analyzedfor detecting non-cardiac noise based on an analysis of the cardiacelectrical signal using the techniques disclosed herein. An atrialtachyarrhythmia episode may be rejected based on non-cardiac noisedetection.

More generally, the disclosed techniques may be used in any device thatis configured to determine a heart rate from sensed cardiac electricalsignals, such as fitness trackers, watches, or other heart ratemonitors. When the cardiac electrical signal is corrupted by non-cardiacnoise, the determined heart rate may be incorrect, e.g., overestimated,due to the non-cardiac noise signals being falsely sensed as cardiacevents.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 configured to sense cardiac electrical events and delivercardiac electrical stimulation therapies according to one example. FIG.1A is a front view of ICD system 10 implanted within patient 12. FIG. 1Bis a side view of ICD system 10 implanted within patient 12. ICD system10 includes an ICD 14 connected to an extra-cardiovascular electricalstimulation and sensing lead 16. FIGS. 1A and 1B are described in thecontext of an ICD system 10 capable of providing high voltage CV/DFshocks, and in some examples cardiac pacing pulses, in response todetecting a cardiac tachyarrhythmia. However, the techniques disclosedherein for detecting non-cardiac noise may be implemented in othercardiac devices configured for sensing cardiac events and, for example,determining a cardiac event interval or rate, for use in determining thecardiac rate or rhythm and controlling a cardiac electrical stimulationtherapy.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). Housing 15 may be used as an active can electrode for use indelivering CV/DF shocks or other high voltage pulses delivered using ahigh voltage therapy circuit. In other examples, housing 15 may beavailable for use in delivering unipolar, low voltage cardiac pacingpulses and/or for sensing cardiac electrical signals in combination withelectrodes carried by lead 16. In other instances, the housing 15 of ICD14 may include a plurality of electrodes on an outer portion of thehousing. The outer portion(s) of the housing 15 functioning as anelectrode(s) may be coated with a material, such as titanium nitride,e.g., for reducing post-stimulation polarization artifact.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,cardiac electrical signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.

Elongated lead body 18 has a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead body 18 includes defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. For example, either of electrodes 24 and 26 may be used asa sensing electrode in a sensing vector for sensing cardiac electricalsignals and determining a need for an electrical stimulation therapy.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering relatively lowvoltage pacing pulses in some configurations. Electrodes 28 and 30 arereferred to as pace/sense electrodes because they are generallyconfigured for use in low voltage applications, e.g., used as either acathode or anode for delivery of pacing pulses and/or sensing of cardiacelectrical signals, as opposed to delivering high voltage CV/DF shocks.In some instances, electrodes 28 and 30 may provide only pacingfunctionality, only sensing functionality or both.

ICD 14 may obtain cardiac electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing electrode vectors thatinclude combinations of electrodes 24, 26, 28 and/or 30. In someexamples, housing 15 of ICD 14 is used in combination with one or moreof electrodes 24, 26, 28 and/or 30 in a sensing electrode vector.Various sensing electrode vectors utilizing combinations of electrodes24, 26, 28, and 30 and housing 15 are described below for sensing firstand second cardiac electrical signals using respective first and secondsensing electrode vectors that may be selected by sensing circuitryincluded in ICD 14.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. One, two or more pace/senseelectrodes may be carried by lead body 18. For instance, a thirdpace/sense electrode may be located distal to defibrillation electrode26 in some examples. Electrodes 28 and 30 are illustrated as ringelectrodes; however, electrodes 28 and 30 may comprise any of a numberof different types of electrodes, including ring electrodes, short coilelectrodes, hemispherical electrodes, directional electrodes, segmentedelectrodes, or the like. Electrodes 28 and 30 may be positioned at otherlocations along lead body 18 and are not limited to the positions shown.In other examples, lead 16 may include fewer or more pace/senseelectrodes and/or defibrillation electrodes than the example shown here.

In the example shown, lead 16 extends subcutaneously or submuscularlyover the ribcage 32 medially from the connector assembly 27 of ICD 14toward a center of the torso of patient 12, e.g., toward xiphoid process20 of patient 12. At a location near xiphoid process 20, lead 16 bendsor turns and extends superiorly, subcutaneously or submuscularly, overthe ribcage and/or sternum, substantially parallel to sternum 22.Although illustrated in FIG. 1A as being offset laterally from andextending substantially parallel to sternum 22, the distal portion 25 oflead 16 may be implanted at other locations, such as over sternum 22,offset to the right or left of sternum 22, angled laterally from sternum22 toward the left or the right, or the like. Alternatively, lead 16 maybe placed along other subcutaneous or submuscular paths. The path ofextra-cardiovascular lead 16 may depend on the location of ICD 14, thearrangement and position of electrodes carried by the lead body 18,and/or other factors. The techniques disclosed herein are not limited toa particular path of lead 16 or final locations of electrodes 24, 26, 28and 30.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18, which may beseparate respective insulated conductors within the lead body 18, areeach electrically coupled with respective defibrillation electrodes 24and 26 and pace/sense electrodes 28 and 30. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry, suchas a therapy delivery circuit and/or a sensing circuit, of ICD 14 viaconnections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy delivery circuit within ICD 14 to one ormore of defibrillation electrodes 24 and 26 and/or pace/sense electrodes28 and 30 and transmit sensed electrical signals produced by thepatient's heart 8 from one or more of defibrillation electrodes 24 and26 and/or pace/sense electrodes 28 and 30 to the sensing circuit withinICD 14.

The lead body 18 of lead 16 may be formed from a non-conductivematerial, including silicone, polyurethane, fluoropolymers, mixturesthereof, and/or other appropriate materials, and shaped to form one ormore lumens within which the one or more conductors extend. Lead body 18may be tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. Lead body 18 may be formed having a preformeddistal portion 25 that is generally straight, curving, bending,serpentine, undulating or zig-zagging.

In the example shown, lead body 18 includes a curving distal portion 25having two “C” shaped curves, which together may resemble the Greekletter epsilon, “c.” Defibrillation electrodes 24 and 26 are eachcarried by one of the two respective C-shaped portions of the lead bodydistal portion 25. The two C-shaped curves are seen to extend or curvein the same direction away from a central axis of lead body 18, alongwhich pace/sense electrodes 28 and 30 are positioned. Pace/senseelectrodes 28 and 30 may, in some instances, be approximately alignedwith the central axis of the straight, proximal portion of lead body 18such that mid-points of defibrillation electrodes 24 and 26 arelaterally offset from pace/sense electrodes 28 and 30.

Other examples of extra-cardiovascular leads including one or moredefibrillation electrodes and one or more pacing and sensing electrodescarried by curving, serpentine, undulating or zig-zagging distal portionof the lead body 18 that may be implemented with the techniquesdescribed herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety. The techniques disclosed herein are notlimited to any particular lead body design, however. In other examples,lead body 18 is a flexible elongated lead body without any pre-formedshape, bends or curves.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. Example techniquesfor detecting a tachyarrhythmia are described in conjunction with theflow charts presented herein.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF) using a therapy deliveryelectrode vector which may be selected from any of the availableelectrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliver ATP inresponse to VT detection and in some cases may deliver ATP prior to aCV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. If ATP does notsuccessfully terminate VT or when VF is detected, ICD 14 may deliver oneor more CV/DF shocks via one or both of defibrillation electrodes 24 and26 and/or housing 15. ICD 14 may deliver the CV/DF shocks usingelectrodes 24 and 26 individually or together as a cathode (or anode)and with the housing 15 as an anode (or cathode). ICD 14 may generateand deliver other types of electrical stimulation pulses such aspost-shock pacing pulses or bradycardia pacing pulses using a pacingelectrode vector that includes one or more of the electrodes 24, 26, 28,and 30 and the housing 15 of ICD 14.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferiorly from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space. FIGS. 1A and 1B are illustrative in nature andshould not be considered limiting of the practice of the techniquesdisclosed herein.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor52, memory 53, display 54, user interface 56 and telemetry unit 58.Processor 52 controls external device operations and processes data andsignals received from ICD 14. Display 54, which may include a graphicaluser interface, displays data and other information to a user forreviewing ICD operation and programmed parameters as well as cardiacelectrical signals retrieved from ICD 14.

User interface 56 may include a mouse, touch screen, key pad or the liketo enable a user to interact with external device 40 to initiate atelemetry session with ICD 14 for retrieving data from and/ortransmitting data to ICD 14, including programmable parameters forcontrolling cardiac event sensing and therapy delivery. Telemetry unit58 includes a transceiver and antenna configured for bidirectionalcommunication with a telemetry circuit included in ICD 14 and isconfigured to operate in conjunction with processor 52 for sending andreceiving data relating to ICD functions via communication link 42.

Communication link 42 may be established between ICD 14 and externaldevice 40 using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi,or Medical Implant Communication Service (MICS) or other RF orcommunication frequency bandwidth or communication protocols. Datastored or acquired by ICD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may alternatively be embodied as a homemonitor or hand held device. External device 40 may be used to programcardiac signal sensing parameters, cardiac rhythm detection parametersand therapy control parameters used by ICD 14. At least some controlparameters used in detecting noise according to techniques disclosedherein may be programmed into ICD 14 using external device 40 in someexamples.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22(see FIG. 2C). The distal portion 25 of lead 16 may extend along theposterior side of sternum 22 substantially within the loose connectivetissue and/or substernal musculature of anterior mediastinum 36. A leadimplanted such that the distal portion 25 is substantially withinanterior mediastinum 36, may be referred to as a “substernal lead.”

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallysuch that distal portion 25 of lead 16 is underneath/below the ribcage32 in addition to or instead of sternum 22. In other examples, thedistal portion 25 of lead 16 may be implanted in otherextra-cardiovascular, intra-thoracic locations, including the pleuralcavity or around the perimeter of and adjacent to the pericardium 38 ofheart 8.

In the various example implant locations of extracardiovascular lead 16and electrodes 24, 26, 28 and 30, cardiac signals sensed by ICD 14 maybe contaminated by skeletal muscle myopotentials and/or environmentalEMI. In some cases, the repetitive motion or sustained musclecontractions may produce episodes of myopotential noise pulsescontaminated the cardiac electrical signal sensed by ICD 14. Some noisepulses may be oversensed as cardiac events, e.g., R-waves, resulting ina false heart rate being determined. In some instances, the noise may goundetected by a noise detection algorithm, even when some noise pulsesare oversensed as cardiac events. If the noise detection algorithm doesnot detect the noise, but some noise pulses are being oversensed ascardiac events, the heart rate may be overestimated. A falsetachyarrhythmia detection may be made, or bradycardia pacing may bewithheld when it is actually needed. When the noise is not detected bythe noise detection algorithm, falsely sensed cardiac events may gounchecked. Accordingly, the techniques disclosed herein provideimprovements in non-cardiac noise detection by including a gainadjustment that may be used to increase the amplitude of noise pulses inthe cardiac electrical signal to allow the noise pulses to be morereadily detected as described below. The increased gain signal may beused for detecting noise without altering the gain of a signal used forsensing cardiac events. In this way, noise pulses present in the cardiacelectrical signal may be more reliably detected and identified as noiseso that corrective action may be taken if a noise pulse is falselysensed (oversensed) as a cardiac event.

FIG. 3 is a conceptual diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 3 ) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver therapy asneeded according to programmed therapy delivery algorithms and controlparameters. ICD 14 may be coupled to an extra-cardiovascular lead, suchas lead 16 carrying extra-cardiovascular electrodes 24, 26, 28, and 30,for delivering electrical stimulation pulses to the patient's heart andfor sensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, cardiac electrical signal sensing circuit 86, and telemetrycircuit 88. A power source 98 provides power to the circuitry of ICD 14,including each of the components 80, 82, 84, 86, and 88 as needed. Powersource 98 may include one or more energy storage devices, such as one ormore rechargeable or non-rechargeable batteries. The connections betweenpower source 98 and each of the other components 80, 82, 84, 86 and 88are to be understood from the general block diagram of FIG. 3 , but arenot shown for the sake of clarity. For example, power source 98 may becoupled to one or more charging circuits included in therapy deliverycircuit 84 for charging holding capacitors included in therapy deliverycircuit 84 that are discharged at appropriate times under the control ofcontrol circuit 80 for producing electrical pulses according to atherapy protocol. Power source 98 is also coupled to components ofcardiac electrical signal sensing circuit 86, such as sense amplifiers,analog-to-digital converters, switching circuitry, etc. as needed.

The circuits shown in FIG. 3 represent functionality included in ICD 14and may include any discrete and/or integrated electronic circuitcomponents that implement analog and/or digital circuits capable ofproducing the functions attributed to ICD 14 herein. Functionalityassociated with one or more circuits may be performed by separatehardware, firmware or software components, or integrated within commonhardware, firmware or software components. For example, cardiac eventsensing and detection of noise for rejecting sensed events orwithholding detection of a tachyarrhythmia based on cardiac eventintervals may be performed cooperatively by sensing circuit 86 andcontrol circuit 80 and may include operations implemented in a processoror other signal processing circuitry included in control circuit 80executing instructions stored in memory 82 and control signals such asblanking and timing intervals and sensing threshold amplitude signalssent from control circuit 80 to sensing circuit 86.

The various circuits of ICD 14 may include an application specificintegrated circuit (ASIC), an electronic circuit, a processor (shared,dedicated, or group) and memory that execute one or more software orfirmware programs, a combinational logic circuit, state machine, orother suitable components or combinations of components that provide thedescribed functionality. The particular form of software, hardwareand/or firmware employed to implement the functionality disclosed hereinwill be determined primarily by the particular system architectureemployed in the ICD and by the particular detection and therapy deliverymethodologies employed by the ICD. Providing software, hardware, and/orfirmware to accomplish the described functionality in the context of anymodern implantable cardiac device system, given the disclosure herein,is within the abilities of one of skill in the art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such asrandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80and/or other ICD components to perform various functions attributed toICD 14 or those ICD components. The non-transitory computer-readablemedia storing the instructions may include any of the media listedabove.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 andthe housing 15, which may function as a common or ground electrode or asan active can electrode for delivering CV/DF shock pulses or cardiacpacing pulses.

Cardiac electrical signal sensing circuit 86 (also referred to herein as“sensing circuit” 86) may be selectively coupled to electrodes 28, 30and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector together or in combination with one or more ofelectrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabledto selectively receive cardiac electrical signals from at least twosensing electrode vectors from the available electrodes 24, 26, 28, 30,and housing 15 in some examples. At least two cardiac electrical signalsfrom two different sensing electrode vectors may be receivedsimultaneously by sensing circuit 86 in some examples. Sensing circuit86 may monitor one or both of the cardiac electrical signalssimultaneously for sensing cardiac electrical events and/or producingdigitized cardiac signal waveforms for analysis by control circuit 80.For example, sensing circuit 86 may include switching circuitry forselecting which of electrodes 24, 26, 28, 30, and housing 15 are coupledto a first sensing channel 83 and which electrodes are coupled to asecond sensing channel 85 of sensing circuit 86.

Each sensing channel 83 and 85 may be configured to amplify, filter anddigitize the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor detecting cardiac electrical events, such as R-waves or performingother signal analysis. The cardiac event detection circuitry withinsensing circuit 86 may include one or more sense amplifiers, filters,rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), timers or other analog or digital components asdescribed further in conjunction with FIG. 4 . A cardiac event sensingthreshold may be automatically adjusted by sensing circuit 86 under thecontrol of control circuit 80, based on timing intervals and sensingthreshold values determined by control circuit 80, stored in memory 82,and/or controlled by hardware, firmware and/or software of controlcircuit 80 and/or sensing circuit 86.

Upon detecting a cardiac event based on a sensing threshold crossing,first sensing channel 83 may produce a sensed event signal, such as anR-wave sensed event signal, that is passed to control circuit 80. Thesensed event signal is used by control circuit 80 to trigger storage ofa time segment of a cardiac electrical signal for processing andanalysis for detecting noise in the cardiac electrical signal asdescribed below. In some examples, sensing circuit 86 senses at leastone cardiac electrical signal received by a sensing electrode vectorselected from the available electrodes, e.g., electrodes 24, 26, 28, 30and housing 15, for detecting R-waves and buffering multiple cardiacelectrical signal segments, where each cardiac electrical signal segmentcorresponds to a detected R-wave, for processing and analysis fordetecting noise. A single cardiac electrical signal sensed by firstsensing channel 83 may be used for both R-wave detection and analysis ofcardiac electrical signal segments for noise detection. In otherexamples, R-waves are detected from the first cardiac electrical signalsensed by the first sensing channel 83 and segments of a second cardiacelectrical signal sensed by the second sensing channel 85 may bebuffered, with each segment corresponding to an R-wave sensed from thefirst cardiac electrical signal. Noise detection may be based on theanalysis of the second cardiac electrical signal segments. The secondcardiac electrical signal may be received via a sensing electrode paircoupled to the second sensing channel 85 different than the sensingelectrode pair coupled to the first sensing channel 83 for sensing thanthe first cardiac electrical signal and/or may be received by the samesensing electrode pair but processed differently, e.g., filtereddifferently, by the second sensing channel 85 to produce a secondcardiac electrical signal sensed by sensing circuit 86 different thanthe first cardiac electrical signal.

Memory 82 may be configured to store a predetermined number of cardiacelectrical signal segments in a circulating buffer under the control ofcontrol circuit 80, e.g., at least one, two, three or other number ofcardiac electrical signal segments. Each segment may be written tomemory 82 over a time interval extending before and after an R-wavesensed event signal produced by the first sensing channel 83. Controlcircuit 80 may access stored cardiac electrical signal segments whenconfirmation of R-waves sensed by the first sensing channel 83 isrequired based on the detection of a predetermined number oftachyarrhythmia intervals, which may precede tachyarrhythmia detection.In some examples, an R-wave sensed by the first sensing channel 83 maybe rejected when an associated cardiac electrical signal segmentbuffered from the second sensing channel 85 (including the time of thesensed R-wave) is classified as a noise segment. In other examples, thesensed R-wave may be used to determine an RRI that may be counted as atachyarrhythmia interval but if a tachyarrhythmia detection criterion issatisfied, tachyarrhythmia detection may be withheld when a thresholdnumber of cardiac electrical signal segments are classified as noisesegments. Methods for classifying a cardiac electrical signal segment asa noise segment are described below, e.g., in conjunction with FIGS. 5-8.

The R-wave sensed event signals are also used by control circuit 80 fordetermining RRIs for detecting tachyarrhythmia and determining a needfor therapy. An RRI is the time interval between two consecutivelysensed R-waves and may be determined between consecutive R-wave sensedevent signals received by control circuit 80 from sensing circuit 86.For example, control circuit 80 may include a timing circuit 90 fordetermining RRIs between consecutive R-wave sensed event signalsreceived from sensing circuit 86 and for controlling various timersand/or counters used to control the timing of therapy delivery bytherapy delivery circuit 84. Timing circuit 90 may additionally set timewindows such as morphology template windows, morphology analysis windowsor perform other timing related functions of ICD 14 includingsynchronizing cardioversion shocks or other therapies delivered bytherapy delivery circuit 84 with sensed cardiac events.

Control circuit 80 is also shown to include a tachyarrhythmia detector92 configured to analyze signals received from sensing circuit 86 fordetecting tachyarrhythmia. Tachyarrhythmia detector 92 may detecttachyarrhythmia based on cardiac events detected from a sensed cardiacelectrical signal meeting tachyarrhythmia criteria, such as a thresholdnumber of detected cardiac events occurring at a tachyarrhythmiainterval. In some examples, a tachyarrhythmia detection based on thethreshold number of detected cardiac events each occurring at atachyarrhythmia interval may be rejected based on non-cardiac noisebeing detected using the techniques disclosed herein. Tachyarrhythmiadetector 92 may be implemented in control circuit 80 as hardware,software and/or firmware that processes and analyzes signals receivedfrom sensing circuit 86 for detecting VT and/or VF. In some examples,the timing of R-wave sense event signals received from sensing circuit86 is used by timing circuit 90 to determine RRIs between sensed eventsignals. Tachyarrhythmia detector 92 may include comparators andcounters for counting RRIs determined by timing circuit 90 that fallinto various rate detection zones for determining a ventricular rate orperforming other rate- or interval-based assessment of R-wave sensedevent signals for detecting and discriminating VT and VF.

For example, tachyarrhythmia detector 92 may compare the RRIs determinedby timing circuit 90 to one or more tachyarrhythmia detection intervalzones, such as a tachycardia detection interval zone and a fibrillationdetection interval zone. RRIs falling into a detection interval zone arecounted by a respective VT interval counter or VF interval counter andin some cases in a combined VT/VF interval counter included intachyarrhythmia detector 92. The VF detection interval threshold may beset to 300 to 350 milliseconds (ms), as examples. For instance, if theVF detection interval is set to 320 ms, RRIs that are less than 320 msare counted by the VF interval counter. When VT detection is enabled,the VT detection interval may be programmed to be in the range of 350 to420 ms, or 400 ms as an example. In order to detect VT or VF, therespective VT or VF interval counter is required to reach a threshold“number of intervals to detect” (NID).

As an example, the NID to detect VT may require that the VT intervalcounter reaches 32 VT intervals counted out of the most recent 32consecutive RRIs. The NID required to detect VF may be programmed to 18VF intervals out of the most recent 24 consecutive RRIs or 30 VFintervals out 40 consecutive RRIs, as examples. When a VT or VF intervalcounter reaches an NID threshold, a ventricular tachyarrhythmia may bedetected by tachyarrhythmia detector 92. The NID may be programmable andrange from as low as 12 to as high as 40, with no limitation intended.VT or VF intervals may be detected consecutively or non-consecutivelyout of the specified number of most recent RRIs. In some cases, acombined VT/VF interval counter may count both VT and VF intervals anddetect a tachyarrhythmia episode based on the fastest intervals detectedwhen a specified NID is reached.

Tachyarrhythmia detector 92 may be configured to perform other signalanalysis for determining if other detection criteria are satisfiedbefore detecting VT or VF, such as R-wave morphology criteria, onsetcriteria, and noise and oversensing rejection criteria. Examples ofparameters that may be determined from cardiac electrical signalsreceived from sensing circuit 86 for detecting noise that may causewithholding of a VT or VF detection are described below.

To support these additional analyses, sensing circuit 86 may pass adigitized electrocardiogram (ECG) signal to control circuit 80 formorphology analysis performed by tachyarrhythmia detector 92 fordetecting and discriminating heart rhythms. A cardiac electrical signalfrom the selected sensing vector, e.g., from first sensing channel 83and/or the second sensing channel 85, may be passed through a filter andamplifier, provided to a multiplexer and thereafter converted to amulti-bit digital signal by an analog-to-digital converter, all includedin sensing circuit 86, for storage in memory 82. Memory 82 may includeone or more circulating buffers to temporarily store digital cardiacelectrical signal segments for analysis performed by control circuit 80.Control circuit 80 may be a microprocessor-based controller that employsdigital signal analysis techniques to characterize the digitized signalsstored in memory 82 to recognize and classify the patient's heart rhythmemploying any of numerous signal processing methodologies for analyzingcardiac signals and cardiac event waveforms, e.g., R-waves. As describedbelow, processing and analysis of digitized signals may includedetermining signal features for detecting noise present in the cardiacelectrical signal(s). When noise is detected, a tachyarrhythmiadetection based on RRIs may be withheld to inhibit a tachyarrhythmiatherapy. Alternatively, a tachyarrhythmia therapy may be withheld inresponse to a tachyarrhythmia detection made when noise is alsodetected.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices such as one or more high voltage capacitorsand/or low voltage capacitors, and switching circuitry that controlswhen the capacitor(s) are discharged across a selected pacing electrodevector or CV/DF shock vector. Charging of capacitors to a programmedpulse amplitude and discharging of the capacitors for a programmed pulsewidth may be performed by therapy delivery circuit 84 according tocontrol signals received from control circuit 80. Control circuit 80 mayinclude various timers or counters that control when cardiac pacingpulses are delivered. For example, timing circuit 90 may includeprogrammable digital counters set by a microprocessor of the controlcircuit 80 for controlling the basic pacing time intervals associatedwith various pacing modes or ATP sequences delivered by ICD 14. Themicroprocessor of control circuit 80 may also set the amplitude, pulsewidth, polarity or other characteristics of the cardiac pacing pulses,which may be based on programmed values stored in memory 82.

In response to detecting VT or VF, control circuit 80 may schedule atherapy and control therapy delivery circuit 84 to generate and deliverthe therapy, such as ATP and/or CV/DF therapy. Therapy can be generatedby initiating charging of high voltage capacitors via a chargingcircuit, both included in therapy delivery circuit 84. Charging iscontrolled by control circuit 80 which monitors the voltage on the highvoltage capacitors, which is passed to control circuit 80 via a chargingcontrol line. When the voltage reaches a predetermined value set bycontrol circuit 80, a logic signal is generated on a capacitor full lineand passed to therapy delivery circuit 84, terminating charging. A CV/DFpulse is delivered to the heart under the control of the timing circuit90 by an output circuit of therapy delivery circuit 84 via a controlbus. The output circuit may include an output capacitor through whichthe charged high voltage capacitor is discharged via switchingcircuitry, e.g., an H-bridge, which determines the electrodes used fordelivering the cardioversion or defibrillation pulse and the pulse waveshape.

In some examples, the high voltage therapy circuit configured to deliverCV/DF shock pulses can be controlled by control circuit 80 to deliverpacing pulses, e.g., for delivering ATP, post shock pacing pulses orventricular pacing pulses. In other examples, therapy delivery circuit84 may include a low voltage therapy circuit for generating anddelivering pacing pulses for a variety of pacing needs.

It is recognized that the methods disclosed herein for detecting noisemay be implemented in a medical device that is used for monitoringcardiac electrical signals by sensing circuit 86 and control circuit 80without having therapy delivery capabilities or in a pacemaker thatmonitors cardiac electrical signals and delivers cardiac pacingtherapies by therapy delivery circuit 84, without high voltage therapycapabilities, such as cardioversion/defibrillation shock capabilities.

Control parameters utilized by control circuit 80 for sensing cardiacevents and controlling therapy delivery may be programmed into memory 82via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication or other communication protocols as describedabove. Under the control of control circuit 80, telemetry circuit 88 mayreceive downlink telemetry from and send uplink telemetry to externaldevice 40.

FIG. 4 is a diagram of circuitry included in sensing circuit 86 havingfirst sensing channel 83 and second sensing channel 85 according to oneexample. First sensing channel 83 may be selectively coupled viaswitching circuitry included in sensing circuit 86 to a first sensingelectrode vector including at least one electrode carried byextra-cardiovascular lead 16 for receiving a first cardiac electricalsignal. In some examples, first sensing channel 83 may be coupled to asensing electrode vector that is a short bipole, having a relativelyshorter inter-electrode distance or spacing than the second electrodevector coupled to second sensing channel 85. First sensing channel 83may be coupled to a sensing electrode vector that is approximatelyvertical (when the patient is in an upright position) or approximatelyaligned with the cardiac axis to increase the likelihood of a relativelyhigh R-wave signal amplitude relative to P-wave signal amplitude. In oneexample, the first sensing electrode vector may include pace/senseelectrodes 28 and 30. In other examples, the first sensing electrodevector coupled to sensing channel 83 may include a defibrillationelectrode 24 and/or 26, e.g., a sensing electrode vector betweenpace/sense electrode 28 and defibrillation electrode 24 or betweenpace/sense electrode 30 and either of defibrillation electrodes 24 or26. In still other examples, the first sensing electrode vector may bebetween defibrillation electrodes 24 and 26.

Sensing circuit 86 includes second sensing channel 85 for sensing asecond cardiac electrical signal in some examples. For instance, secondsensing channel 85 may receive a raw cardiac electrical signal from asecond sensing electrode vector, for example from a vector that includesone electrode 24, 26, 28 or 30 carried by lead 16 paired with housing15. Second sensing channel 85 may be selectively coupled to othersensing electrode vectors, which may form a relatively longer bipolehaving an inter-electrode distance or spacing that is greater than thesensing electrode vector coupled to first sensing channel 83 in someexamples. The second sensing electrode vector may be, but notnecessarily, approximately orthogonal to the first channel sensingelectrode vector in some cases. For instance, defibrillation electrode26 and housing 15 may be coupled to second sensing channel 85 to providethe second cardiac electrical signal. As described below, the secondcardiac electrical signal received by second sensing channel 85 via along bipole may be used by control circuit 80 for analysis and detectionof noise. The long bipole coupled to second sensing channel 85 mayprovide a relatively far-field or more global cardiac signal compared tothe relatively shorter bipole coupled to the first sensing channel. Inother examples, any vector selected from the available electrodes, e.g.,electrodes 24, 26, 28, 30 and/or housing 15, may be included in asensing electrode vector coupled to second sensing channel 85. Thesensing electrode vectors coupled to first sensing channel 83 and secondsensing channel 85 may be different sensing electrode vectors, which mayhave no common electrodes or only one common electrode but not both.

In other examples, however, the sensing electrode vectors coupled to thefirst sensing channel 83 and the second sensing channel 85 may be thesame sensing electrode vector. The two sensing channels 83 and 85 mayinclude different filters, amplifiers, or other signal processingcircuitry such that two different signals are sensed by the respectivesensing channels 83 and 85 and different analyses may be performed onthe two signals. For example, the first sensing channel 83 may sense afirst cardiac electrical signal by filtering and processing the receivedcardiac electrical signal for detecting R-waves in response to an R-wavesensing threshold crossing for determining RRIs. The second sensingchannel 85 may sense a second cardiac electrical signal different thanthe first by filtering and processing the received cardiac electricalsignal for passing signal segments to control circuit 80 for analysisfor noise detection. The first sensing channel 83 may apply relativelynarrower band pass filtering, and the second sensing channel 85 mayapply relatively wider band pass filtering and notch filtering toprovide two different sensed cardiac electrical signals, received viathe same sensing electrode vector in some examples.

In the illustrative example shown in FIG. 4 , the electrical signalsdeveloped across the first sensing electrode vector, e.g., electrodes 28and 30, are received by first sensing channel 83 and electrical signalsdeveloped across the second sensing electrode vector, e.g., electrodes26 and housing 15, are received by second sensing channel 85. Thecardiac electrical signals are provided as differential input signals tothe pre-filter and pre-amplifier 62 or 72, respectively, of firstsensing channel 83 and second sensing channel 85. Non-physiological highfrequency and DC signals may be filtered by a low pass or bandpassfilter included in each of pre-filter and pre-amplifiers 62 and 72, andhigh voltage signals may be removed by protection diodes included inpre-filter and pre-amplifiers 62 and 72. Pre-filter and pre-amplifiers62 and 72 may amplify the pre-filtered signal by a gain of between 10and 100, and in one example a gain of 17.5, and may convert thedifferential signal to a single-ended output signal passed toanalog-to-digital converter (ADC) 63 in first sensing channel 83 and toADC 73 in second sensing channel 85. Pre-filters and amplifiers 62 and72 may provide anti-alias filtering and noise reduction prior todigitization.

ADC 63 and ADC 73, respectively, convert the first cardiac electricalsignal from an analog signal to a first digital bit stream and thesecond cardiac electrical signal to a second digital bit stream. In oneexample, ADC 63 and ADC 73 may be sigma-delta converters (SDC), butother types of ADCs may be used. In some examples, the outputs of ADC 63and ADC 73 may be provided to decimators (not shown), which function asdigital low-pass filters that increase the resolution and reduce thesampling rate of the respective first and second cardiac electricalsignals.

The digital outputs of ADC 63 and ADC 73 are each passed to respectivefilters 64 and 74, which may be digital bandpass filters. The bandpassfilters 64 and 74 may have the same or different bandpass frequencies.For example, filter 64 may have a bandpass of approximately 13 Hz to 39Hz for passing cardiac electrical signals such as R-waves typicallyoccurring in this frequency range. Filter 74 of the second sensingchannel 85 may have a bandpass of approximately 2.5 to 100 Hz. In someexamples, second sensing channel 85 may further include a notch filter76 to filter 60 Hz or 50 Hz noise signals.

The bandpass filtered signal in first sensing channel 83 is passed fromfilter 64 to rectifier 65 to produce a filtered, rectified signal. Firstsensing channel 83 includes an R-wave detector 66 for sensing cardiacevents in response to the first cardiac electrical signal crossing anR-wave sensing threshold. R-wave detector 66 may include anauto-adjusting sense amplifier, comparator and/or other detectioncircuitry that compares the filtered and rectified cardiac electricalsignal to an R-wave sensing threshold in real time and produces anR-wave sensed event signal 68 when the cardiac electrical signal crossesthe R-wave sensing threshold outside of a post-sense blanking interval.The R-wave sensing threshold may be a multi-level sensing threshold asdisclosed in commonly assigned U.S. Pat. No. 10,252,071 (Cao, et al.),incorporated herein by reference in its entirety. Briefly, themulti-level sensing threshold may have a starting sensing thresholdvalue held for a time interval, which may be equal to a tachycardiadetection interval or expected R-wave to T-wave interval, then drops toa second sensing threshold value held until a drop time intervalexpires, which may be 1 to 2 seconds long. The sensing threshold dropsto a minimum sensing threshold, which may correspond to a programmedsensitivity sometimes referred to as the “sensing floor,” after the droptime interval. In other examples, the R-wave sensing threshold used byR-wave detector 66 may be set to a starting value based on the peakamplitude determined during the most recent post-sense blanking intervaland decay linearly or exponentially over time until reaching a minimumsensing threshold. The techniques described herein are not limited to aspecific behavior of the sensing threshold or specific R-wave sensingtechniques. Instead, other decaying, step-wise adjusted or otherautomatically adjusted sensing thresholds may be utilized.

The notch-filtered, digital cardiac electrical signal 78 from secondsensing channel 85 may be passed to memory 82 for buffering a segment ofthe second cardiac electrical signal 78 in response to an R-wave sensedevent signal 68 produced by the first sensing channel 83. In someexamples, the buffered segment of the second cardiac electrical signal78 is rectified by rectifier 75 before being stored in memory 82. Insome cases, both the filtered, non-rectified signal 78 and the rectifiedsignal 79 are passed to control circuit 80 and/or memory 82 for use indetermining features of multiple segments of the second cardiacelectrical signal, where each segment extends over a time interval thatencompasses the time point of an R-wave sensed event signal produced bythe first sensing channel 83.

Second sensing channel 85 may include a filter 77 in some examples.Filter 77 may be a first order derivative filter for receiving the notchfiltered signal from notch filter 76 and producing a first orderdifferential signal 81 (e.g., where the i^(th) sample of the first orderdifferential signal is the difference between the i^(th) sample point ofthe notch filtered signal and the preceding i−1 sample point of thenotch filtered signal). In other examples, a higher order differentialsignal may be output by derivative filter 77, e.g., second order orhigher. In still other examples, derivative filter 77 may be a high passfilter with a sharp cut-off corner frequency, e.g., a 50 Hz, 60 Hz, 80Hz or 100 Hz high pass filter as examples. Filter 77 may be configuredto produce a filtered signal that removes low frequency cardiac eventsignals without removing higher frequency noise pulses, which may be 50Hz or higher. The differential (or filtered) signal 81 may be bufferedin memory 82 and passed to control circuit 80 for processing andanalysis for detecting noise contamination presumed to be present inboth of the first cardiac electrical signal 68 and the second cardiacelectrical signal 78. In some examples, the differential signal 81 maybe rectified by rectifier 75 before buffering in memory 82 forprocessing and analysis by control circuit 80. In other examples, thedifferential signal 81 is buffered in memory 82 without rectifying andis processed and analyzed by control circuit 80 for detecting noisecontamination as described below.

Control circuit 80 is configured to detect tachyarrhythmia based oncardiac events detected from at least one cardiac electrical signalsensed by sensing circuit 86. For example, control circuit 80 may beconfigured to detect tachyarrhythmia when a detection threshold numberof detected cardiac events each occur at a tachyarrhythmia interval.Control circuit 80 may buffer segments of a sensed cardiac electricalsignal in memory 82 and retrieve stored signal segments from memory 82for analysis when a lower threshold number of tachyarrhythmia intervalshave been detected, before the NID tachyarrhythmia detection thresholdis reached. In some examples, RRIs for detecting tachyarrhythmiaintervals are determined from the first cardiac electrical signal sensedby first sensing channel 83, and cardiac electrical signal segments arebuffered from the second cardiac electrical signal received by controlcircuit 80 from second sensing channel 85 for noise analysis when thelower threshold number of tachyarrhythmia intervals is detected.Analysis of the second cardiac electrical signal segments may beperformed for use in detecting non-cardiac noise before the detectionthreshold number of tachyarrhythmia intervals (NID) is reached, asdescribed below. In other examples, a single cardiac electrical signalsensed by sensing circuit 86 is used for sensing R-waves for determiningRRIs and counting tachyarrhythmia intervals and is buffered for storinga cardiac electrical signal segments for use in detecting noise, whereeach buffered segment is associated with one sensed R-wave.

For instance, control circuit 80 may be configured to determine amaximum amplitude from each one of multiple, consecutive cardiacelectrical signal segments for determining if signal to noise criteriaare met. If so, control circuit 80 may increase the signal gain tointentionally increase the amplitude of noise pulses in the cardiacelectrical signal segment. By increasing the signal gain, non-cardiacnoise pulses may be more readily detected as noise. Increasing the gainimproves detection of noise allowing identification of potentialoversensing of noise pulses as R-waves, which can lead totachyarrhythmia intervals being detected. One or more noise metrics maybe determined from the increased gain signal for determining if noisecriteria are met. When a threshold number of cardiac electrical signalsegments meet noise criteria, detection of a tachyarrhythmia based on athreshold number of tachyarrhythmia intervals or other detectioncriterion may be withheld or rejected. In other examples, an RRI that isassociated with a cardiac electrical signal segment determined to benoise contaminated may be ignored and not used in countingtachyarrhythmia intervals. Time segments of the notch-filtered,rectified signal 79 received from second sensing channel 85 may be usedto detect noise segments that may result in withholding atachyarrhythmia detection in some examples.

The configuration of sensing channels 83 and 85 as shown in FIG. 4 isillustrative in nature and should not be considered limiting of thetechniques described herein. The sensing channels 83 and 85 of sensingcircuit 86 may include more or fewer components than illustrated anddescribed in FIG. 4 and some components may be shared between sensingchannels 83 and 85. For example, one or more of pre-filter andpre-amplifiers 62/72, ADC 63/73, and/or filters 64/74 may be sharedcomponents between sensing channels 83 and 85 with a single, sensedsignal output split to two sensing channels for subsequent processingand analysis. Sensing circuit 86 and control circuit 80 includecircuitry configured to perform the functionality attributed to ICD 14in detecting non-cardiac noise and rejecting or withholdingtachyarrhythmia interval or episode detection in response to detectingnon-cardiac noise as disclosed herein.

FIG. 5 is a flow chart 100 of a method for detecting non-cardiac noisein a cardiac electrical signal according to one example. In variousexamples presented herein, non-cardiac noise pulses, such as skeletalmuscle myopotentials, may be oversensed by sensing circuit 86 as R-waves(or other cardiac event signals corresponding todepolarization/repolarization of the cardiac tissue). For example, whena noise pulse crosses an R-wave sensing threshold of R-wave detector 66(FIG. 4 ), a false R-wave sensed event signal 68 may be produced at atachyarrhythmia interval from the most recent preceding R-wave sensedevent signal, leading to a VT or VF interval counter being increased. Itis to be understood, however, that the techniques disclosed herein fordetecting non-cardiac noise may be applied for detecting likelyoversensing of noise when P-waves are being sensed by a sensing circuitof a medical device. In this case, oversensing of non-cardiac noisepulses occurs when a noise pulse in the cardiac electrical signalcrosses the P-wave sensing threshold, causing the sensing circuit toproduce a false P-wave sensed event signal. The false P-wave sensedevent signal may be counted as a tachyarrhythmia interval, toward atrialtachyarrhythmia detection criteria being met. Furthermore, the methodshown in FIG. 5 may be adapted for detecting noise in any electricalsignal sensed by a medical device being used to monitor or detectelectrophysiological event signals. Noise signals that may corrupt theelectrical signal may be detected using the technique of flow chart 100.

At block 102, sensing circuit 86 senses a cardiac event based on acardiac event sensing threshold crossing by a cardiac electrical signal.In some examples, the cardiac electrical signal is received by firstsensing channel via a sensing electrode vector that is a relatively nearfield signal for increasing the likelihood of sensing cardiac events ina desired heart chamber, e.g., ventricular or atrial, withoutoversensing a cardiac event in the adjacent heart chamber, e.g., atrialor ventricular. In one example, cardiac events sensed at block 102 areintended to be R-waves sensed based on an R-wave sensing thresholdcrossing detected by the first sensing channel 83 of sensing circuit 86as described above. In other medical applications, anelectrophysiological event, e.g., corresponding to an action potentialor depolarization or repolarization of excitable tissue, may be sensedfrom an electrical signal sensed by a medical device at block 102.

At block 104, control circuit 80 may buffer a segment of an electricalsignal in memory 82 for noise analysis in response to anelectrophysiological event sensed at block 102. Noise analysis may beperformed by control circuit 80 to reject the sensedelectrophysiological event (or reject or withhold detecting a conditionbased on the sensed electrophysiological event) in response to detectingnoise in the buffered signal segment associated with the sensedelectrophysiological event. Control circuit 80 may buffer the signalsegment in memory 82 at block 104 for further analysis and processing asdescribed below. In some examples, the buffered signal segment is fromthe same signal that the event was detected from at block 102, and inother examples the buffered signal segment is a different signal, e.g.,sensed using a different sensing electrode pair and/or produced bysensing circuit 84 using different filtering, amplification, etc. thanthe first signal.

The buffered segment may be a cardiac electrical signal received fromthe second sensing channel 85, and may be notch-filtered to attenuate50-60 Hz noise. In some examples, the second sensing channel 85 receivesa cardiac electrical signal via a different sensing electrode vector forsensing a second cardiac electrical signal different than the firstcardiac electrical signal used at block 102 for sensing cardiac events.In other examples, the same sensing electrode vector is used forreceiving a single cardiac electrical signal used for both sensingcardiac events and buffering cardiac electrical signal segments used fornoise detection. When the same sensing electrode vector is used,different filtering or other signal processing may be used for producinga first sensed cardiac electrical signal for detecting cardiac events atblock 102 and producing a second sensed cardiac electrical signal forbuffering signal segments at block 104 for noise analysis.

A segment of the second cardiac electrical signal may be buffered over apredetermined time interval that encompasses a time point at which acardiac event was sensed from the first cardiac electrical signal. Forexample, in response to an R-wave sensed event signal 68 received fromthe first sensing channel 83 (see FIG. 4 ), control circuit 80 maybuffer a time segment of the second cardiac electrical signal 78 (andthe rectified signal 79 in some examples) from the second sensingchannel 85 in memory 82. The time segment may extend from a time pointearlier than the time of one R-wave sensing threshold crossing to a timepoint later than the R-wave sensing threshold crossing that caused thefirst sensing channel 83 to generate an R-wave sensed event signal 68.The time segment may be 300 to 500 ms in duration, e.g., 360 ms induration, including sample points preceding and following the one R-wavesensed event signal. For instance, as described in conjunction with FIG.6 , a 360 ms segment may include 92 sample points when the sampling rateis 256 Hz with 24 of the sample points occurring after the R-wave sensedevent signal that triggered the storage of the signal segment and 68sample points extending from the R-wave sensed event signal earlier intime from the R-wave sensed event signal.

At block 106, control circuit 80 determines a metric of signal strength.Determining the metric of signal strength may include determining amaximum signal amplitude during the buffered signal segment associatedwith the sensed electrophysiological event. For example, control circuit80 may determine the signal strength metric as the maximum amplitude ofthe buffered cardiac signal during the signal segment. In one example,control circuit 80 determines the maximum amplitude of the rectifiedsignal segment (or absolute maximum amplitude of a non-rectified signalsegment). As described further below, the maximum signal amplitude ofeach one of multiple buffered signal segments may be determined and thegreatest one of the maximum signal amplitudes may be used as a signalstrength metric for determining if signal to noise criteria are met atblock 110.

At block 108, control circuit 80 determines a metric of the strength oramplitude of possible noise during the buffered signal segment. In oneexample, control circuit 80 determines the noise strength metric bydetermining a differential signal from the buffered signal segment. Forinstance, control circuit 80 may determine a first order differentialsignal by determining the difference between each pair of consecutivesample points of the notch filtered cardiac electrical signal segment.In other examples, a higher order differential signal may be determined,e.g., second order or higher. Alternatively, the differential signal maybe produced by a high pass filter with a sharp corner cut-off frequency,e.g., a cut-off frequency of at least 50 Hz or 60 Hz for example whenthe sampling frequency is 256 Hz. The maximum absolute amplitude of thedifferential signal of the buffered signal segment is determined atblock 108 as a noise strength metric in one example.

At block 110, control circuit 80 determines if signal to noise criteriaare met based on the signal strength metric and the noise strengthmetric. In some examples, control circuit 80 may determine if signal tonoise criteria are met based on the signal strength metric and/or thenoise strength metric determined from multiple signal segments, whichmay be consecutively buffered signal segments. For example the greatestsignal strength metric determined from multiple signal segments and thenoise strength metric determined from the current signal segment may becompared to signal to noise criteria for determining whether the currentsignal segment is detected as a noise segment. The number of signalsegments that control circuit 80 determines the maximum signal strengthmetric from may occur over a predetermined time interval expected toinclude at least one true electrophysiological event e.g., at least onetrue R-wave, as further described below in conjunction with FIG. 6 . Thesignal to noise criteria may include one or more requirements applied toone or more signal strength metrics and/or one or more noise strengthmetrics, individually or in one or more combinations, e.g., as one ormore ratios of a signal strength metric to a noise strength metric.

In some examples, the signal to noise criteria include a signal strengthcriterion and a noise strength criterion. The signal strength criterionmay be applied to the maximum signal strength metric, e.g., the greatestmaximum amplitude determined from one or more preceding, recentlybuffered signal segments. The maximum signal strength metric is anindication of the amplitude of an expected cardiac event signal that mayhave occurred during a preceding signal segment. The noise strengthcriterion may be applied to the noise strength metric of the currentlybuffered signal segment, e.g., the maximum amplitude of the differentialsignal of the currently buffered signal segment. In this example,control circuit 80 identifies the greatest maximum amplitude out ofrecently buffered signal segments as an indication of cardiac eventsignal strength and identifies the maximum amplitude of the differentialsignal of the currently buffered signal segment as an indication of thenoise strength in the currently buffered signal segment. Control circuit80 uses these metrics to determine when signal to noise criteria are metat block 110. In other examples, control circuit 80 may determine aratio of the maximum signal strength metric to the maximum noisestrength metric and compare this ratio to a minimum ratio threshold todetermine when the signal to noise criteria are met.

FIG. 6 is a diagram 200 of a first cardiac electrical signal 202 and asecond cardiac electrical signal 232 illustrating analysis performedaccording to one method for determining if signal to noise criteria aremet for increasing the gain of the differential signal. In the firstcardiac electrical signal 202, two true R-waves 204 and 206 are sensedby the first sensing channel 83 resulting in respective R-wave sensedevents signals 208 and 210. However, due to non-cardiac noise present inthe first cardiac electrical signal 202, two noise pulses 212 and 214are each sensed by R-wave detector 66 resulting in false R-wave sensedevents signals 216 and 218. Each of the R-wave sensed event signals 208,210, 216 and 218, denoted by “VS” to indicate a ventricular sensedevent, trigger buffering of a segment of the second cardiac electricalsignal 232 over a respective time interval 233, 235, 241, and 243.

Each time interval 233, 235, 241, and 243 may extend before and afterthe time of the respective R-wave sensed event signal 208, 210, 216, and218 such that the associated buffered second cardiac electrical signalsegment includes sample points before and after the time of theassociated R-wave sensed event signal. As indicated above, in oneexample, each time interval 233, 235, 241 and 243 is about 360 ms induration so that the respective second cardiac electrical signal segmentbuffered over the time interval includes 92 sample points when thesampling rate is 256 Hz with 24 of the sample points occurring after theR-wave sensed event signal that triggered the storage of the signalsegment and 68 sample points extending from the R-wave sensed eventsignal earlier in time from the R-wave sensed event signal. Each cardiacelectrical signal segment corresponding to a respective time interval233, 235, 241 or 243 encompasses the time of a single one R-wave sensedevent signal.

Control circuit 80 determines a maximum absolute amplitude 234 of thebuffered cardiac electrical signal segment over time interval 233,maximum absolute amplitude 242 of the buffered cardiac electrical signalsegment over time interval 241, and maximum absolute amplitudes 244 and236 over the cardiac electrical signal segments buffered over respectivetime intervals 243 and 235. This process of determining maximumamplitudes of each buffered second cardiac electrical signal segment maycorrespond to determining a metric of R-wave signal strength at block106 of FIG. 5 described above. These maximum amplitudes 234, 242, 244and 236 may be stored in a first-in-first-out buffer so that the maximumamplitude of each one of multiple second cardiac electrical signalsegments is buffered in a rolling, first-in-first-out buffer. The buffermay store a predetermined number of maximum amplitudes, e.g., 3 to 10maximum amplitudes corresponding to the most recent 3 to 10 most recentcardiac electrical signal segments and associated R-wave sensed eventsignals.

In other examples, the maximum amplitude buffer may store each of themaximum amplitudes determined from second cardiac electrical signalsegments that are buffered in response to each R-wave sensed eventsignal during at least a predetermined time interval 252, 254. Thepredetermined time interval, referred to as a maximum amplitude buffertime interval, may be one to two seconds long in some examples. Themaximum amplitude buffer time interval is selected so that at least onetrue R-wave is expected to occur over the time interval 252 or 254. Forexample, when the time interval 252, 254 is 1.2 seconds, a true R-wave(e.g., R-wave 234 or R-wave 236) is expected to occur within the 1.2seconds when the true heart rate is as low as 50 beats per minute. Themaximum amplitude buffer time interval may correspond to or be based ona lower pacing rate time interval or an expected resting heart rate ofthe patient. Each maximum amplitude determined from each cardiacelectrical signal segment that is buffered during a 1.2 second, 1.5second, 2.0 second or other selected time interval may be stored in amaximum amplitude buffer. In this case, the number of maximum amplitudesstored in the maximum amplitude buffer may be variable since a differentnumber of R-wave sensed event signals, and thus a different number ofcardiac electrical signal segments, may occur during each fixed,predetermined time interval 252, 254 depending on how many non-cardiacnoise pulses are oversensed and the actual ventricular rate. When aventricular blanking interval is set to 150 ms and a maximum amplitudebuffer time interval is set to 1.2 seconds, the maximum possible numberof R-wave sensed event signals is eight, resulting in eightcorresponding cardiac electrical signal segments and eight maximumamplitudes stored in the maximum amplitude buffer.

In the example shown, a maximum amplitude buffer time interval 252,which may be 1.2 seconds long or other selected time interval, extendsfrom a most recent cardiac electrical signal segment buffered over timeinterval 243 (or from the associated R-wave sensed event signal 218) andincludes any preceding cardiac electrical signal segments (or maximumamplitudes) that have been buffered within the maximum amplitude buffertime interval 252. In other examples, the maximum amplitude buffer inmemory 82 is configured to store up to a fixed number of maximumamplitudes, e.g., eight maximum amplitudes, on a first in first outbasis, each with a corresponding timestamp. When a maximum amplitude isdetermined for a given cardiac electrical signal segment, the maximumamplitude may be buffered in memory 82 with a time stamp. The maximumamplitudes stored in the maximum amplitude buffer with a time stamp thatoccurs within the maximum amplitude buffer time interval 252 from thecurrent maximum amplitude time stamp may be evaluated for identifying alikely R-wave from among the maximum amplitudes.

For each cardiac electrical signal segment, therefore, the maximumamplitudes stored with a time stamp that occurs within the maximumamplitude buffer time interval earlier than the current maximumamplitude (or current buffered signal segment) may be identified. In theexample of maximum amplitude buffer time interval 252, the maximumamplitude 244 and the preceding maximum amplitudes 234 and 242 occurringearlier but within the maximum amplitude buffer time interval 252 areevaluated for identifying the maximum amplitude that has the greatestprobability of being an R-wave during the maximum amplitude buffer timeinterval. In other examples, maximum amplitudes may be evaluated over amaximum amplitude buffer time interval starting from a maximumamplitude, R-wave sensed event signal or beginning of a cardiacelectrical signal segment and going forward in time rather than backwardin time as described here. However this may delay determining whetherthe current cardiac electrical signal segment meets the signal to noisecriteria by the one to two second maximum amplitude buffer timeinterval, which may result in a later detection of noise than goingbackward in time from the current maximum amplitude.

The greatest maximum amplitude 234 of the buffered maximum amplitudesduring time interval 252 is identified by control circuit 80. Thisgreatest maximum amplitude 234 out of the buffered maximum amplitudes234, 242 and 244 during the one to two second maximum amplitude buffertime interval 252 is presumably a true R-wave amplitude (correspondingto R-wave 204 in this case) and unlikely to be a relatively loweramplitude non-cardiac noise pulse, e.g., caused by skeletal musclemyopotentials. The two lower maximum amplitudes 242 and 244 may benon-cardiac noise pulses that are oversensed as R-waves. This greatestmaximum amplitude 234 out of multiple buffered signal segments may bedetermined as a signal strength metric by control circuit 80, e.g., atblock 106 of FIG. 5 . As such, determining the signal strength metricmay require determining a maximum signal amplitude from multipleconsecutive buffered signal segments.

The next R-wave sensed event signal 210 triggers buffering of the nextcardiac electrical signal segment (over time interval 235) and themaximum amplitude 236 during the next cardiac electrical signal segmentis buffered in the first-in-first-out maximum amplitude buffer,replacing the oldest maximum amplitude 234. The other maximum amplitudes242 and 244 occurring during the most recent preceding cardiacelectrical signal segments (associated with time intervals 241 and 243respectively), which are within the fixed predetermined time interval254, remain in the maximum amplitude buffer. Now, the greatest maximumamplitude stored in the maximum amplitude buffer is the maximumamplitude 236, presumably a true R-wave amplitude (corresponding toR-wave 206) because it is the highest amplitude stored in the maximumamplitude buffer.

In this way, the maximum amplitude of each buffered cardiac electricalsignal segment occurring during the moving time interval, as denoted byintervals 252 and 254, is buffered so that a likely true R-waveamplitude can be identified over each time interval 252, 254 as anindication of the R-wave signal strength. The greatest maximum amplitudemay be determined from the moving time interval 252, 254 and compared toa signal strength threshold at block 110 of FIG. 5 to determine if afirst criterion of the signal to noise criteria is met. In one example,the signal strength threshold is set to a percentage or portion of theADC range of the sensing channel. In the example of rectified cardiacelectrical signal segments being buffered from the second cardiacelectrical signal sensed by the second sensing channel 85, the ADC 73may have a range of 127 ADC units, for example. The greatest maximumamplitude may be compared to 30%, 40%, 50% or other percentage orfraction of the maximum ADC range. In one example, the greatest maximumamplitude is compared to one-third of the ADC range of 127 units or 42ADC units. When the greatest absolute maximum amplitude is greater than42 ADC units, the R-wave signal strength may satisfy one criterion ofthe signal to noise criteria applied at block 110 of FIG. 5 . In otherexamples, the signal strength threshold may be set based on previouslyconfirmed R-wave amplitudes, the mean sample point amplitude over thecardiac electrical signal segment or another time interval, or based onanother reference amplitude. The signal strength threshold may beselected such that a signal strength metric that is greater than thesignal strength threshold likely corresponds to a relatively high R-waveamplitude or true electrophysiological event being sensed by the medicaldevice.

In addition to determining whether the signal strength criterion is metat block 110, control circuit 80 may determine whether a noise strengthcriterion is met at block 110 of FIG. 5 . In one example, controlcircuit 80 determines the noise strength metric by determining a firstorder differential signal of the current cardiac electrical signalsegment. Using time interval 252 in FIG. 6 as an example, the firstorder differential signal of the most recent cardiac electrical signalsegment (corresponding to time interval 243) is determined. The maximumamplitude of this differential signal is determined and compared to anoise strength threshold. The noise strength threshold may also bedefined as a percentage or fraction of the ADC range. In one example,the maximum amplitude of the differential signal of the most recentcardiac electrical signal segment in the time interval 252 is comparedto about 10% (or other percentage) of the ADC range, which is a lowerpercentage or fraction than the signal threshold applied to the greatestmaximum amplitude for determining if a signal strength criterion is met.For instance, when the ADC range is 127 ADC units, the noise strengththreshold may be 13 ADC units. When the maximum amplitude of thedifferential signal of the current segment 243 is less than the noisestrength threshold of 13 ADC units, the noise strength criterion of thesignal to noise criteria applied at block 110 is met.

In some examples, the lowest maximum amplitude buffered in the first infirst out buffer or the maximum amplitude of the currently bufferedsignal may be assumed to be an oversensed noise signal and used as thenoise strength metric. However, without filtering lower frequencies fromthe buffered signal segment corresponding to the frequency of a trueelectrophysiological event signal for removing a possible trueelectrophysiological event signal (e.g., a true R-wave signal) beforedetermining the noise strength metric, the lowest maximum amplitude maybe a true event signal. For example, the lowest maximum amplitude may bea low amplitude R-wave or fibrillation wave that should not be presumedto be a noise pulse or rejected as possible noise.

When both the signal strength criterion and the noise strength criterionare met at block 110 of FIG. 5 , control circuit 80 determines that thesignal to noise criteria are satisfied. Generally, the signal strengthcriterion is met when a presumed R-wave amplitude of a recent cardiacelectrical signal segment is greater than a predetermined portion of theADC range or other selected signal strength threshold. The noisestrength criterion is met when the maximum amplitude of the currentdifferential signal segment is less than a predetermined portion of theADC range or other selected noise strength threshold. In other examples,control circuit 80 may determine the ratio of the maximum signalstrength metric to the current noise strength metric and compare thisexpected signal to noise ratio to a minimum acceptable signal to noiseratio threshold. When the ratio of the maximum signal strength metric tothe current noise strength metric is greater than the minimum ratiothreshold, control circuit 80 may determine that the signal to noisecriteria are satisfied.

When the signal to noise criteria are satisfied, control circuit 80increases, e.g., doubles, the gain of the cardiac electrical signalbeing used for detecting noise at block 112. For example, controlcircuit 80 may adjust the gain of the differential signal of the currentcardiac electrical signal segment 243 at block 112 before determining anoise metric from the differential signal segment at block 114. It is tobe understood that the gain increase may be applied to the cardiacelectrical signal segment before or after determining the differentialsignal of the segment. The gain of a high pass filtered signal, firstorder or higher order differential signal, or any cardiac electricalsignal segment being used by control circuit 80 for determining a noisemetric may be adjusted at block 112 to increase the amplitude of noisepulses that may be present in the signal. When the signal to noisecriteria are not met at block 110, control circuit 80 does not adjustthe gain of the signal being used for determining a noise metric and mayadvance directly to block 114 without changing the signal gain.

Control circuit 80 determines the noise metric at block 114 from thecurrent signal segment for detecting the segment as a noise segment (ornot a noise segment). In one example, control circuit 80 may determine anoise pulse count as the noise metric. The noise pulse count may bedetermined by counting pulses defined by consecutive zero crossings ofthe differential signal segment as described below in conjunction withFIG. 7 . In other examples, control circuit 80 may determine the noisemetric by counting the number of “wiggles” or oscillations of the signalover the signal segment, counting the number of inflection points in thesignal segment, counting the number of peaks, counting the number ofcrossings of a threshold, determining an integral or summation ofrectified sample point amplitudes over the signal segment, determiningan average amplitude over the signal segment, determining the highfrequency content of the signal, detecting an episode or time intervalof frequencies continuously above a noise frequency threshold, or othermetric that is correlated to the number and/or amplitude of noise pulsesin the signal segment. In other examples, a noise detector may beincluded in sensing circuit 86 or control circuit 80 configured todetect noise pulses based on amplitude content and/or frequency contentof the signal. For example, a noise detector may detect sustained timeintervals (e.g. as low as 100 ms time intervals or less, 100 to 500 mstime intervals, or longer time intervals) of high frequency contentcorrelated to noise corruption in the cardiac electrical signal. Afterincreasing the gain of the signal segment used for detecting noise, avariety of noise detection techniques may be utilized by the medicaldevice for detecting noise pulses from the increased gain signal.

In the next cardiac electrical signal segment over time interval 235,the maximum amplitude of the differential signal is likely to exceed thenoise strength threshold due to the presence of the large, true R-wavesignal associated with peak amplitude 236. In this case, the maximumamplitude of the current differential signal is greater than the noisestrength threshold. Thus, the signal to noise criteria for the timeinterval 254 are not satisfied because the noise strength criterion isnot met. The gain of the differential signal over time interval 235 isleft unchanged, and control signal 80 advances to block 114 to determinethe noise metric from the differential signal of the cardiac electricalsignal segment over time interval 235 without the gain adjustment.Determination of a noise metric at block 114 in accordance with oneexample is described below in conjunction with FIG. 7 .

Control circuit 80 determines when noise criteria are met at block 116based on the determined noise metric and may classify the currentcardiac electrical signal segment as a noise segment at block 118. Whenthe noise criteria are unmet at block 116, the current cardiacelectrical signal segment is not classified as a noise segment. Theprocess may return to block 102 to wait for the next sensed event signalto repeat the process for the next electrical signal segment buffered inresponse to the next sensed event signal.

FIG. 7 is a flow chart 300 of a method for determining a noise metricand classifying a cardiac electrical signal segment as a noise segmentor non-noise segment according to some examples. The noise metric may bedetermined from the first order (or other higher order) differentialsignal determined from the notch filtered cardiac electrical signalsegment buffered from the second sensing channel 85 in some examples.The differential signal is analyzed to determine the noise metric atblock 114 of FIG. 5 without changing the gain when the signal to noisecriteria are not satisfied at block 110 of FIG. 5 as described above.The noise metric is determined from the differential signal afterincreasing the gain of the differential signal, e.g., doubling the gain,when the signal to noise criteria are satisfied at block 110 of FIG. 5 .

At block 302, control circuit 80 may determine zero crossings of asegment of the differential signal. The differential signal may bereceived and buffered from second sensing channel 85 as described aboveor determined by control circuit 80 from the cardiac electrical signalsegment buffered from second sensing channel 85. The zero crossings maybe determined by identifying a pair of sample points of the differentialsignal straddling a zero crossing, including one sample point (positiveor negative) immediately prior to the zero crossing and a second samplepoint (negative or positive) immediately after the zero crossing. Onesample point of this pair of sample points having the smallest absolutevalue is identified and set to a zero amplitude by control circuit 80 todemarcate the zero crossing and define the ending point of one signalpulse and starting point of the next consecutive signal pulse. In someinstances one of the sample points of the pair of sample points at azero crossing may have a zero amplitude, and the second sample point maybe positive or negative. The zero amplitude sample point may be selectedas the zero crossing defining the ending point of one signal pulse andthe starting point of the next signal pulse. The zeroed sample pointsseparate and define consecutive pulses of the differential signal asillustrated in FIG. 8 .

FIG. 8 is a graphical representation 400 of a cardiac electrical signalsegment 401 and corresponding first order differential signal 410.Cardiac electrical signal segment 401 is buffered over time segment 402in response to an R-wave sensed event signal as generally describedabove in conjunction with FIG. 6 . The R-wave sensed event signal may beassociated with a true R-wave or an oversensed noise signal. The firstorder differential signal 410 of the cardiac electrical signal segment401 is determined by hardware, firmware and/or software included insensing circuit 86 and/or control circuit 80 for analysis for detectingnoise contamination. A pair of sample points 404 a and 404 b includesthe last sample point 404 a before a zero crossing of the first orderdifferential signal 410 and the earliest sample point 404 b after thezero crossing. The absolute values of the sample points 404 a and 404 bare compared by control circuit 80, and the smallest absolute valuesample point is set to zero amplitude at block 302 of FIG. 7 todemarcate consecutive pulses of the differential signal 410.

With continued reference to FIGS. 7 and 8 , the differential signal isrectified at block 304 of FIG. 7 to produce the rectified differentialsignal 420 shown in FIG. 8 . Each pulse of rectified differential signal420, for example pulse 422, is defined by two consecutive zeroed samplepoints 424 and 426. Zeroed sample point 424, for example, may be thesmallest absolute amplitude sample point 404 a or 404 b set to a valueof zero to mark the end of the preceding pulse and the onset of pulse422.

Control circuit 80 determines whether the signal to noise criteria aremet at block 306, e.g., according to the techniques described inconjunction with FIGS. 5 and 6 . The gain of rectified differentialsignal 420 is increased at block 308, e.g., doubled or increased byanother selected factor, to produce an increased gain signal, e.g.,increased gain signal 430 of FIG. 8 when the signal to noise criteriaare met.

When the signal to noise criteria are not met, the rectifieddifferential signal 420 without a gain adjustment is analyzed fordetecting a noise segment. The process of flow chart 300 advances fromblock 306 to block 310 without increasing the gain of the rectifieddifferential signal at block 308 before performing noise analysis. It isnoted that the gain of differential signal 420 is increased to increasethe amplitude of non-cardiac noise pulses to facilitate counting ofnoise pulses and detection of noise corruption. The gain of the cardiacelectrical signal is not increased at block 308 to facilitate R-wavesensing. R-wave sensing from the selected cardiac electrical signal isperformed separately and independently of gain adjustments applied todifferential signal 420 for noise detection. The R-wave may be sensedfrom a cardiac electrical signal before the determination that signal tonoise criteria are met and before making a gain adjustment to thedifferential signal.

At block 310, control circuit 80 determines a pulse detection thresholdamplitude. The pulse detection threshold amplitude may be determinedbased on a maximum amplitude of whichever of the rectified differentialsignal 420 or increased gain signal 430 is being analyzed for noisedetection based on the signal to noise criteria. For example, themaximum peak amplitude 428 of rectified differential signal 420 (whensignal to noise criteria are unmet) or maximum peak amplitude 438 ofincreased gain signal 430 (when signal to noise criteria are met) may bedetermined by determining the maximum amplitude of the respectivedifferential signal 428 or 438 over the time segment 402. The pulsedetection threshold amplitude may be set to a fraction or percentage ofthe greatest maximum amplitude (428 or 438), e.g., one half (or otherfraction) of the greatest maximum amplitude. In one example, the pulsedetection threshold amplitude is set to one-eighth of the greatestmaximum amplitude of the rectified first order differential signal (420or 430).

At block 312 of FIG. 7 , the pulse detection threshold amplitude iscompared to a suspected noise threshold amplitude. When the pulsedetection threshold amplitude, a fraction or percentage of the maximumamplitude of the differential signal segment, is less than a suspectednoise threshold, control circuit 80 may classify the segment as anon-noise segment at block 320. In other examples, the maximum amplitudeof the differential signal segment may be compared directly to asuspected noise threshold amplitude. The current cardiac electricalsignal segment is determined to not be noise contaminated at block 320when the pulse detection threshold amplitude is less than a suspectednoise threshold. The suspected noise threshold may be set to a value of1 ADC unit in some examples but may be set to a value of 5 ADC units orless or another selected threshold in various examples.

Referring again to FIG. 8 , the noise in cardiac electrical signalsegment 401, observed as the noise pulses in the rectified differentialsignal 420 without increased gain, may go undetected when the maximumpulse amplitude 428 is less than a threshold amplitude (or the pulsedetection threshold amplitude set to a portion of the maximum amplitude428 is less than the suspected noise threshold). However, by increasingthe gain of the differential signal 420 to produce the increased gainsignal 430, the pulse detection threshold amplitude set based on theincreased (e.g., doubled) maximum pulse amplitude 438 may meet or exceedthe suspected noise threshold. This enables the control circuit 80 todetermine that the cardiac electrical signal segment 401 is suspected tocontain noise and enable analysis of the increased gain signal 430 todetermine if the corresponding signal segment 401 is noise corrupted.

When low amplitude myopotential noise is present in the signal segment,for example as represented by the signal pulses of signal segment 401and the first order differential signal 410, oversensing of myopotentialnoise may cause a VT or VF NID to be reached. When this myopotentialnoise is not identified or detected, a false VT or VF detection may bemade leading to therapy delivery. By increasing the gain of the firstorder differential signal when the signal to noise criteria are met, themyopotential noise pulses are more readily detectable, enabling anoise-contaminated signal segment to be identified thereby avoiding afalse VT or VF detection and unnecessary VT or VF therapy.

Referring again to FIG. 7 , when the pulse detection threshold amplitudeis greater than or equal to the suspected noise threshold at block 312,control circuit 80 identifies and counts noise pulses present in thedifferential signal segment to determine if the segment is noisy. Eachpulse of the rectified differential signal 420 (when the signal to noisecriteria are not met) or the increased gain rectified, differentialsignal 430 (when the signal to noise criteria are met), defined by thesample points between two consecutive zeros, may be counted. In someexamples, control circuit 80 may count a pulse as a noise pulse onlywhen the pulse meets noise pulse criteria at block 314.

The noise pulse criteria may include pulse amplitude criteria and/orpulse width criteria. For instance, a pulse may be counted as a noisepulse at block 314 when the pulse has an amplitude greater than thepulse detection threshold amplitude and a pulse width, defined by thenumber of sample points between the consecutive zeros defining thepulse, that is less than or equal to a pulse width threshold. The pulsewidth threshold may be set to a sample point number of six or less butmay be set to other values depending in part on the sampling rate. Whenthe maximum amplitude between consecutive zeros is less than or equal tothe pulse detection threshold amplitude and/or the number of samplepoints between consecutive zeros is greater than the pulse detectionthreshold width, the pulse is not counted as a noise pulse at block 314.

At block 316, control circuit 80 compares the number of pulses meetingnoise pulse criteria and thus counted as noise pulses at block 314 to athreshold count. When the number of noise pulses counted is less thanthe threshold count, the cardiac electrical signal segment from whichthe differential signal was derived is classified as a non-noise segmentat block 320. The current cardiac electrical signal segment isdetermined to not be noise contaminated. The associated R-wave sensedevent signal is presumed to be a true sensed R-wave or at least not anon-cardiac noise signal. This determination based on the second cardiacelectrical signal may be extended to the first cardiac electrical signalsuch that both signals are determined to be clean signals, withoutnon-cardiac noise contamination, when the noise pulse count determinedfrom one cardiac electrical signal is less than the noise thresholdcount. When the number of noise pulses counted equals or exceeds thethreshold count, however, the cardiac electrical signal segment may beclassified as a noise segment at block 322. This classification may beextended to the first cardiac electrical signal such that the R-wavesensed event signal that triggered the buffering of the second cardiacelectrical signal may be a falsely sensed R-wave due to non-cardiacnoise.

In some examples, before classifying the cardiac electrical signalsegment as a noise segment at block 322, control circuit 80 maydetermine if a tachyarrhythmia morphology is present in the cardiacelectrical signal segment at block 318. The presence of atachyarrhythmia morphology may preclude classification of a signalsegment as a noise segment to avoid withholding a tachyarrhythmiadetection due to noise detection when evidence of tachyarrhythmiamorphology is detected in the cardiac electrical signal segment.Evidence of tachyarrhythmia morphology may be detected at block 318based on a gross morphology analysis of the signal segment. Instead ofcounting individual pulses in the differential signal segment, the grossmorphology of the cardiac electrical signal segment, e.g., signalsegment 401, may be analyzed to assess the morphology of the overallwaveform 405 that the noise pulses 407 may be riding one. For instance,control circuit 80 may determine a signal amplitude metric and signalwidth metric of the overall signal segment, e.g., using rectified signalsegment 403 in FIG. 8 . Methods for determining a gross morphologysignal amplitude metric and a gross morphology signal width metric fordetecting evidence of a tachyarrhythmia morphology at block 318 of FIG.7 are described below in conjunction with FIGS. 8-10 . A grossmorphology signal amplitude and signal width that is correlated to asinusoidal-like fibrillation waveform may be detected as evidence of atachyarrhythmia morphology, for example.

Control circuit 80 may classify the cardiac electrical signal segment asa non-noise segment at block 320 when a tachyarrhythmia morphology isdetermined to be present (“yes” branch of block 318), even though thenoise pulse count may meet the threshold count for detecting non-cardiacnoise at block 316. When a tachyarrhythmia morphology is not detected atblock 318 (“no” branch), the cardiac electrical signal segment isdetected as a noise segment at block 322 in response to the noise pulsecount meeting the threshold count at block 316.

FIG. 9 is a flow chart 500 of a method for determining a grossmorphology amplitude metric of a cardiac electrical signal segment fordetecting a tachyarrhythmia morphology according to one example. Eachcardiac electrical signal segment analyzed for determining a grossmorphology metric may be buffered as generally described above inconjunction with FIG. 6 . The method of flow chart 500 may generally beperformed at block 318 of FIG. 7 to analyze a cardiac electrical signalsegment, which may be suspected of being a noise contaminated signal.

At block 502, the second cardiac electrical signal segment stored on atriggered basis in response to an R-wave sensed event signal may berectified. In some examples, a 360 ms segment of the notch-filteredsecond cardiac electrical signal may be rectified by rectifier 75included second sensing channel 85. At block 502, the buffered,rectified signal segment may be retrieved by control circuit 80 frommemory 82. In other examples, a notch-filtered signal segment may bebuffered in memory 82, and control circuit 80 may perform therectification of the stored signal segment at block 502. A rectifiedsignal segment 403 buffered over time segment 402 is shown in FIG. 8 .The rectified signal segment, e.g., segment 403, obtained at block 502may correspond to a signal segment identified as a suspected noisesegment based on the maximum amplitude of the differential signal andthe differential signal noise pulse count as described in conjunctionwith FIG. 7 .

With continued reference to FIGS. 8 and 9 , control circuit 80determines the maximum absolute amplitude 408 of the rectified,notch-filtered signal segment 403 at block 504. The maximum absoluteamplitude 408 may be determined from among all sample points spanningthe selected signal segment 403. As described above, a 360 ms segment ofthe second cardiac electrical signal may include 92 sample points whenthe sampling rate is 256 Hz, with 24 of the sample points occurringafter the R-wave sensed event signal that triggered the storage of thesignal segment and 68 sample points extending from the R-wave sensedevent signal earlier in time from the R-wave sensed event signal.

At block 506, the amplitudes of all sample points of the rectifiedsignal segment are summed, which represents the area 406 of therectified signal segment 403. At block 508, a gross morphology amplitudemetric of the signal segment is determined as a normalized rectifiedamplitude (NRA) based on the maximum absolute amplitude 408 determinedat block 504 and the summed sample point amplitudes (area 406)determined at block 506. In one example, the NRA is determined as apredetermined multiple or weighting of the summation of all sample pointamplitudes of the notch-filtered and rectified signal segment 403normalized by the maximum amplitude 408. For instance, the NRA may bedetermined as four times the summed amplitudes (area 406) divided by themaximum absolute amplitude 408, which may be truncated to an integervalue. This NRA may be determined as a gross morphology amplitude metricat block 318 of FIG. 7 for detecting a tachyarrhythmia morphology basedon the sample points spanning the signal segment that extends before andafter the R-wave sensed event signal. As can be seen in theillustrations of FIG. 8 , the gross morphology amplitude metricdetermined from the maximum amplitude 408 and area 406 represents anamplitude metric of the underlying cardiac signal waveform 405 that theindividual non-cardiac noise pulses 407 may be riding on. As such, thenoise pulse count described in conjunction with FIG. 7 is useful indetecting non-cardiac noise pulses 407 that may be contaminating theoverall cardiac signal waveform 405 whereas the gross morphologyamplitude metric, and the gross morphology width metric described below,are useful in detecting a gross morphology of the underlying cardiacsignal waveform 405 in the signal segment 403 that may correspond to atachyarrhythmia waveform morphology.

For example, the gross morphology amplitude metric determined as theweighted area 406 divided by the maximum amplitude 408 may be inverselycorrelated to the probability of the signal segment sample points beingat a baseline amplitude during the time segment 402. The higher thegross morphology amplitude metric is, the lower the probability that thesignal is at a baseline amplitude at any given time point during thetime segment 402. A relatively low probability that the signal 403 is atbaseline during the time segment 402 may be correlated to atachyarrhythmia morphology, e.g., a ventricular fibrillation morphology,which may resemble a sinusoidal signal. When the gross morphologyamplitude metric exceeds a threshold value the more likely the cardiacelectrical signal segment has a tachyarrhythmia morphology. When thegross morphology amplitude metric is less than the threshold value, thehigher the probability that the signal is at a baseline amplitude at agiven time point during the time segment 402 of the signal segment 403.A relatively higher probability of a signal sample point being atbaseline during the time segment 402 may be correlated to a relativelynarrow R-wave signal occurring during the signal segment, or no trueR-wave being present, with baseline amplitude portions of the signalsegment occurring before and after the R-wave sensed event signal. Assuch, when a tachyarrhythmia morphology is not detected and the noisepulse count reaches a threshold count value, as described above, theR-wave sensed event signal associated with the cardiac electrical signalsegment may correspond to a non-cardiac noise pulse rather than a trueR-wave.

When the gross morphology amplitude metric is greater than apredetermined threshold, “yes” branch of block 510, evidence of anunderlying large amplitude cardiac signal that may correspond to atachyarrhythmia morphology is detected at block 516. In this case,detection of the tachyarrhythmia morphology precludes detecting thesignal segment as a noise segment as described above in conjunction withFIG. 7 . When the NRA is less than or equal to the NRA threshold (“no”branch of block 510), a tachyarrhythmia morphology is not detected atblock 512. The segment may be detected as a noise segment, depending onthe analysis of the pulse count described above in conjunction with FIG.7 . The NRA threshold for detecting a tachyarrhythmia morphology appliedat block 510 may be set between 100 and 150, and is set to 125 in someexamples, such as when 92 sample points are summed and multiplied by aweighting factor of four and normalized by the maximum absoluteamplitude. The NRA threshold to detect a tachyarrhythmia morphology inthe cardiac electrical signal segment may depend on various factors suchas the amplification and number of sample points summed, themultiplication or weighting factor of the summed sample points, etc.

FIG. 10 is a flow chart 550 of a method for detecting a tachyarrhythmiamorphology in a cardiac electrical signal segment based on a grossmorphology signal width metric according to one example. The process offlow chart 550 may be performed by control circuit 80 for determining agross morphology signal width metric at block 318 of FIG. 7 . Blocks 502and 504 correspond to identically-numbered blocks described above inconjunction with FIG. 9 . The notch-filtered, rectified cardiacelectrical signal segment, e.g., signal segment 403 shown in FIG. 8 ,determined at block 502 may be used to determine the maximum absoluteamplitude 408 of the signal segment 403 at block 504.

With continued reference to FIGS. 8 and 10 , control circuit 80determines a pulse amplitude threshold 412 at block 552 based on themaximum absolute amplitude 408 determined at block 504. This pulseamplitude threshold 412 may be used for identifying a signal pulsehaving a maximum signal width out of all signal pulses occurring duringthe time segment 402 of the cardiac electrical signal segment 403. Forexample, the pulse amplitude threshold 412 used for determining thegross morphology signal width metric may be set to half the maximumabsolute amplitude 408 of the rectified, notch-filtered signal segment403.

At block 554, control circuit 80 determines the signal width for allsignal pulses of the second cardiac electrical signal segment 403. Eachsignal pulse in the signal segment 403 may be identified by identifyingtwo consecutive zero amplitude or baseline amplitude sample points ofthe rectified signal segment 403. All signal pulses between twoconsecutive baseline amplitude sample points are identified. The signalpulses may be identified from the non-rectified signal segment 401 toenable signal pulses to be identified between zero-crossings, in someexamples. The signal width of each identified signal pulse is determinedas the number of sample points (or corresponding time interval) betweenthe pair of consecutive baseline amplitude sample points (or zerocrossings). The absolute maximum amplitude of each rectified signalpulse is determined at block 556. All signal pulses of the rectifiedsignal segment 403 that have an absolute maximum amplitude that isgreater than or equal to the pulse amplitude threshold 412 areidentified at block 558. For example, all signal pulses having a maximumamplitude that is at least half the maximum absolute amplitude 408determined at block 504 are identified at block 558. Control circuit 80determines the maximum signal pulse width at block 560 of all identifiedsignal pulses. The number of sample points spanning each identifiedsignal pulse are counted and compared to determine the maximum signalpulse width out of all signal pulses identified at block 558 as havingan amplitude that is at least the pulse amplitude threshold 412. Themaximum pulse width, e.g., pulse width 414 in FIG. 8 , is identified atblock 560 and is determined as the gross morphology signal width metricfor detecting a tachyarrhythmia morphology (e.g., at block 318 of FIG. 7). This maximum signal pulse width 414 is expected to be the pulse widthof the underlying cardiac signal waveform 405.

This gross morphology signal width metric may be correlated to theprobability of the signal segment 401 having a tachyarrhythmiamorphology. For example, a relatively high gross morphology signal widthmetric may be evidence of a tachyarrhythmia morphology, such as arelatively wide ventricular fibrillation wave. Conversely, a relativelylow gross morphology signal width metric may be evidence of a relativelynarrow, true R-wave occurring during the time segment 402 of the cardiacelectrical signal segment 401 or absence of a true cardiac signal. Whena relatively wide signal pulse is not detected from the rectifiedcardiac electrical signal segment 403, an oversensed non-cardiac noisepulse may be present and may have triggered the buffering of the cardiacsignal segment 401.

Control circuit 80 compares the maximum pulse width 414 identified atblock 560 to a pulse width threshold at block 562. In one example, thepulse width threshold is set to 20 sample points when the sampling rateis 256 Hz. When the maximum signal pulse width is less or equal to thewidth threshold, control circuit 80 does not detect a tachyarrhythmiamorphology at block 564. A maximum signal pulse width that is less thanor equal to the width threshold may correspond to a true, relativelynarrow R-wave, e.g., during a sinus rhythm or to a non-cardiac noisepulse. Control circuit 80 may detect a noise segment at block 322 ofFIG. 7 when the maximum signal pulse width is less than or equal to thewidth threshold (tachyarrhythmia morphology not detected at block 318)and at least a threshold number of pulses are counted from thedifferential signal (block 316 of FIG. 7 ).

When the maximum pulse width is greater than the width threshold atblock 562, the relatively wide maximum signal pulse width may bedetected as evidence of a tachyarrhythmia waveform morphology at block564. Evidence of the tachyarrhythmia morphology in signal segment 401precludes detection of a noise segment, even when a threshold number ofnoise pulses are counted from the differential signal 410 derived fromthe cardiac electrical signal segment 401. When the tachyarrhythmiamorphology is detected based on the gross morphology signal widthmetric, the signal segment may not be detected as a noise segment.

In some cases, e.g., when only noise pulses are present in the cardiacelectrical signal segment, no signal pulses having a maximum amplitudegreater than the pulse amplitude threshold may be identified at block558 of FIG. 10 . In this case, the gross morphology width metric is notdetermined, and a tachyarrhythmia morphology is not detected. Thecardiac electrical signal segment may be detected as a noise segmentbased on the noise pulse count.

In various examples, both the gross morphology amplitude metric and thegross morphology signal width metric may be determined (at block 318 ofFIG. 7 according to the techniques of FIG. 9 and FIG. 10 , respectively)and compared to respective tachyarrhythmia morphology thresholds. Insome examples, both of the gross morphology amplitude metric and thegross morphology signal width metric may be required to be less than orequal to the respective tachyarrhythmia morphology threshold for thetachyarrhythmia morphology to not be detected, enabling a noise segmentto be detected. When either one of the gross morphology amplitude or thegross morphology signal width is greater than the respectivetachyarrhythmia morphology threshold value, evidence of atachyarrhythmia morphology may be detected in the cardiac electricalsignal segment at block 318 of FIG. 7 , precluding detection of a noisesegment. In other examples, both of the gross morphology amplitude andsignal width metric may be required to be greater than the respectivetachyarrhythmia morphology threshold value in order to detect atachyarrhythmia morphology and preclude detection of a noise segment. Ifone of the gross morphology metrics, e.g., the amplitude or widthmetric, is less than or equal to the respective tachyarrhythmiamorphology threshold, the tachyarrhythmia morphology criteria may not bemet at block 318, allowing the signal segment to be detected as a noisesegment at block 322.

The gross morphology amplitude metric determined by the method of FIG. 9and the gross morphology signal width metric determined by the method ofFIG. 10 may be used in combination to detect evidence of atachyarrhythmia morphology at block 318 of FIG. 7 to prevent detectionof a noise segment as described above. A segment of the cardiacelectrical signal that has a relatively high gross morphology amplitudemetric and/or relatively high gross morphology signal width metric isevidence of a tachyarrhythmia morphology and is not counted as a noisesegment that may lead to withholding of a tachyarrhythmia detection andsubsequent therapy to maintain a high sensitivity to tachyarrhythmiadetection.

FIG. 11 is a flow chart 600 of a method for controlling tachyarrhythmiadetection and therapy by a medical device according to one example. Atblock 602, control circuit 80 of ICD 14 may determine if criteria forenabling signal analysis for detecting noise segments are met. In oneexample, the criteria for enabling analysis for noise detection requiresa threshold number of tachyarrhythmia intervals. For instance, athreshold number of VT and/or VF intervals less than the NID required todetect VT or VF may trigger signal analysis for detection of noisecontaminated signal segments. For instance, three, five, eight or otherselected number of RRIs falling into the VT and/or VF interval zones maybe required to enable signal analysis for noise detection at block 602.

In other examples, noise analysis criteria may be met when a patientactivity signal indicates that the patient is engaged in physicalactivity. For example, a patient activity metric may be determined froman accelerometer included in ICD 14 or another physiological sensorsignal. When the patient activity metric indicates that the patient isengaged in physical activity above a resting or predetermined thresholdlevel, the noise analysis may be enabled at block 602.

In still other examples, a threshold change in heart rate based onR-wave sensed event signals may meet noise analysis criteria at block602. An increase in heart rate, e.g., based on RRIs less than apredetermined threshold interval, a threshold decrease in a median RRIor other heart rate metric, may be an indication that noise pulses arebeing oversensed. As such, in some cases criteria for enabling noiseanalysis based on heart rate may not require RRIs falling into atachyarrhythmia interval zone. An increase in heart rate that occursquickly or a heart rate above a sub-tachyarrhythmia threshold rate maycause noise analysis criteria to be met at block 602.

The noise analysis criteria may require a combination of two or morecriteria in some examples. For instance, a threshold heart rate ortachyarrhythmia interval count and a threshold patient activity levelmay be required in order to enable noise analysis at block 602. When thecriteria for enabling signal analysis for noise detection are met atblock 602, control circuit 80 waits for the next R-wave sensed eventsignal from sensing circuit 86 at block 606 and buffers a correspondingcardiac electrical signal segment at block 608. In some examples, thecardiac electrical signal segment is buffered from the second cardiacelectrical signal from second sensing channel 85 when the R-wave issensed by the first sensing channel 83 from the first cardiac electricalsignal. In other examples, the cardiac electrical signal segment may bebuffered from the same cardiac electrical signal from which the R-wavewas sensed.

At block 610, control circuit 80 performs the noise analysis to classifythe cardiac electrical signal segment as a noise segment or not a noisesegment. Control circuit 80 may classify the cardiac electrical signalsegment as noise or non-noise according to any of the example techniquesdescribed above in conjunction with FIGS. 5-10 . In general, a noisemetric such as a noise pulse count, inflection count, integral orsummation of sample point amplitudes, high frequency content or othernoise metric correlated to the number, frequency and/or amplitude ofnoise pulses may be determined from the differential signal of thecardiac electrical signal. The noise metric may be determined afterincreasing the gain of the differential signal when signal to noisecriteria are met as described above. When the noise metric exceeds anoise threshold value, the segment may be classified as a noise segment.When a tachyarrhythmia morphology is detected, e.g., according to thetechniques described in conjunction with FIGS. 8-10 , the segment may beclassified as a non-noise segment, such that the noise segmentclassification is withheld when the noise metric exceeds the noisesegment threshold but a tachyarrhythmia morphology is detected.

Control circuit 80 updates a noise segment count at block 612. Afirst-in-first-out buffer in memory 82 may set a flag indicating theclassification of each cardiac electrical signal segment analyzed. Thebuffer may store a flag value (e.g., 1=noise segment and 0=non-noisesegment) for each of a predetermined number of cardiac electrical signalsegments. For example, the buffer may store the classification of eachof six, eight, ten, twelve or other selected number of consecutivecardiac electrical signal segments analyzed on a first-in-first-outbasis. An X of Y counter may be implemented in control circuit 80 thatis updated with each new signal segment classification.

At block 614, control circuit 80 may determine when one or moretachyarrhythmia detection criterion are met. In some examples,tachyarrhythmia detection criteria applied at block 610 may include aninterval-based criterion, such as a required NID being reached by the VTinterval counter, VF interval counter, or a combined VT/VF intervalcounter of tachyarrhythmia detection circuit 92. In other examples, thetachyarrhythmia detection criteria determined to be met or unmet atblock 610 may be based on QRS waveform morphology meetingmorphology-based criteria or a combination of interval or rate-basedcriteria and morphology-based criteria.

When no tachyarrhythmia detection criteria are met at block 614, controlcircuit 80 may return to block 602 to continue performing the signalanalysis for noise detection as long as the criteria for enabling noisedetection remains satisfied at block 602. When the criteria for enablingsignal analysis for noise detection becomes unmet (e.g., patientactivity, heart rate, and/or the VT and/or VF interval count falls belowa respective threshold for enabling noise detection), the count of thenoise segments may be cleared at block 604. For example, a bufferstoring classifications of signal segments as noise or non-noisesegments may be cleared or reset to all non-noise segment values atblock 604.

When at least one criterion for detecting tachyarrhythmia is determinedto be satisfied at block 614, control circuit 80 may compare the currentnoise segment count to withhold criteria at block 616. For example, whenthe number of noise segments counted reaches or exceeds a withholdthreshold, the withhold criteria may be met, and the tachyarrhythmiadetection based on at least one satisfied tachyarrhythmia detectioncriterion is withheld by control circuit 80 at block 620. VT or VF isnot detected by control circuit 80 at block 620 when the NID is reached,for example, and a threshold number of cardiac electrical signalsegments have been classified as noise segments. In some examples, asingle noise segment may meet withhold criteria. When control circuit 80detects a noise segment, the associated sensed R-wave may be rejected,resulting in the NID not being reached.

A tachyarrhythmia therapy that is programmed to be delivered in responseto the VT or VF detection is not scheduled or delivered. In otherexamples, the VT or VF detection may be made in response totachyarrhythmia detection criteria being satisfied at block 614, but thetherapy may be withheld when the number of noise segments meets orexceeds the withhold criteria. When the withhold criteria are not met atblock 616, e.g., when the noise segment count is less than a withholdthreshold, the tachyarrhythmia is detected at block 618 by controlcircuit 80 based on the tachyarrhythmia detection criterion being met.Therapy, e.g., ATP and/or CV/DF shock, may be delivered in response tothe tachyarrhythmia detection.

FIG. 12 is a flow chart 650 of an alternative method for controllingtachyarrhythmia detection and therapy by a medical device. In FIG. 12 ,identically numbered blocks correspond to functions described above inconjunction with FIG. 11 for like-numbered blocks. In the process ofFIG. 12 , control circuit 80 classifies each cardiac electrical signalsegment as noise or non-noise at block 610 when the noise analysiscriteria are met at block 602.

After classifying the cardiac electrical signal segment at block 610,control circuit 80 may be configured to ignore the R-wave sensed eventsignal associated with the cardiac electrical signal segment. When thesegment is classified as a noise segment, control circuit 80 may ignorethe R-wave sensed event signal without determining an RRI in someexamples. In the example shown in FIG. 12 , control circuit maydetermine if the RRI ending with the R-wave sensed event signalassociated with the current cardiac electrical signal segment is atachyarrhythmia interval at block 652.

If so, control circuit 80 determines if the current cardiac electricalsignal segment is classified as a noise segment at block 654. If thesignal segment associated with the R-wave sensed event signal ending atachyarrhythmia is classified as a noise segment (“yes” branch of block654), control circuit 80 does not count the tachyarrhythmia interval asa VT or VF interval at block 656. The process returns to block 602. Ifthe RRI is a tachyarrhythmia interval and the signal segment is notclassified as a noise segment (“no” branch of block 654), the RRI iscounted as a tachyarrhythmia interval at block 658.

When at least one tachyarrhythmia detection criterion is met at block614, e.g., the NID is reached by the VT or VF interval counter, the VTor VF is detected at block 618 and a programmed therapy is delivered atblock 618. In this way, a tachyarrhythmia detection is effectivelywithheld or not made in response to an RRI that is tachyarrhythmiainterval but is associated with an R-wave sensed event signalcorresponding to a cardiac electrical signal segment classified as anoise segment.

FIG. 13 is a flow chart 700 of a method performed by ICD 14 fordetecting non-cardiac noise and rejecting a ventricular tachyarrhythmiadetection in response to detecting noise according to another example.At blocks 702 and 704, two different cardiac electrical signals may bereceived by sensing circuit 86. In some examples, two different sensingelectrode vectors may be selected by sensing circuit 86 for receiving afirst cardiac electrical signal by a first sensing channel 83 and asecond cardiac electrical signal by a second sensing channel 85,respectively. The two sensing electrode vectors may be selected byswitching circuitry included in sensing circuit 86 under the control ofcontrol circuit 80. In some examples, the two sensing electrode vectorsare programmed by a user and retrieved from memory 82 by control circuit80 and passed to sensing circuit 86 as vector selection control signals.

In some examples, the first sensing electrode vector selected forsensing the first cardiac electrical signal at block 702 may be arelatively short bipole, e.g., between electrodes 28 and 30 or betweenelectrodes 28 and 24 of lead 16 or other electrode combinations asdescribed above. The relatively short bipole may include electrodes thatare in relative close proximity to each other and to the ventricularheart chambers to provide sensing of a relatively “near-field”ventricular signal for sensing R-waves compared to a second sensingvector selected at block 704. The first sensing electrode vector may bea vertical sensing vector (with respect to an upright or standingposition of the patient) or approximately aligned with the cardiac axisfor maximizing the amplitude of R-waves in the first cardiac electricalsignal for reliable R-wave sensing. The first sensing electrode vector,however, is not limited to any particular interelectrode spacing ororientation and may be selected as any available electrode pair.

The second sensing electrode vector used to receive a second cardiacelectrical signal at block 704 may be a relatively longer bipole havingan inter-electrode distance that is greater than the first sensingelectrode vector. For example, the second sensing electrode vector maybe selected as the vector between one of the pace sense electrodes 28 or30 and ICD housing 15, one of defibrillation electrodes 24 or 26 andhousing 15 or other combinations of one electrode along the distalportion of the lead 16 and the housing 15. This second sensing electrodevector may be orthogonal or almost orthogonal to the first sensingelectrode vector in some examples, but the first and second sensingvectors are not required to be orthogonal vectors. The second sensingelectrode vector may receive a relatively more global or far-fieldcardiac electrical signal compared to the first sensing electrodevector. The second cardiac electrical signal received by the secondsensing channel 85 at block 304 may be analyzed by control circuit 80for detecting noise corruption of both of the first and second cardiacelectrical signals. In other examples, the first and second cardiacelectrical signals sensed at blocks 702 and 704 may be received from thesame sensing electrode vector, such that a single cardiac electricalsignal is received by the sensing circuit 86, but the raw, receivedsignal may be processed by two different sensing channels 83 and 85 ofsensing circuit 86 having different filtering and/or other signalprocessing features to sense two different cardiac electrical signals,one used by the first sensing channel 83 for detecting R-waves and onesensed by the second sensing channel 85 for detecting noise andperforming tachyarrhythmia morphology analysis. In still other examples,a single cardiac signal is used for sensing R-waves and buffered fordetecting noise corruption of the cardiac signal.

Sensing circuit 86 may produce an R-wave sensed event signal at block706 in response to the first sensing channel 83 detecting an R-wavesensing threshold crossing by the first cardiac electrical signal. TheR-wave sensed event signal may be passed to control circuit 80. Inresponse to the R-wave sensed event signal, down-going “yes” branch ofblock 706, control circuit 80 is triggered at block 708 to store asegment of the second cardiac electrical signal received from the secondsensing channel 85 over a predetermined time interval. Segments of thesecond cardiac electrical signal may be stored in a circulating bufferof memory 82 configured to store multiple sequential segments, wherestorage of each segment is triggered by an R-wave sensed event signalproduced by the first sensing channel 83. A digitized segment of thesecond cardiac electrical signal may be 100 to 500 ms long, for example,including sample points before and after the time of the R-wave sensedevent signal. The segment of the second cardiac electrical signal may ormay not be centered in time on the R-wave sensed event signal receivedfrom sensing circuit 86. For instance, the segment may extend 100 msafter the R-wave sensed event signal and be 200 to 500 ms in durationsuch that the segment extends from about 100 to 400 ms before the R-wavesensed event signal to 100 ms after. In other examples, the segment maybe centered on the R-wave sensed event signal or extend a greater numberof sample points after the R-wave sensed event signal than before. Inone example, the buffered segment of the cardiac electrical signal is atleast 50 sample points obtained at a sampling rate of 256 Hz, or about200 ms. In another example, the buffered segment is at least 92 samplepoints, or approximately 360 ms, sampled at 256 Hz and is available foranalysis for detecting noise present in the cardiac electrical signalsegment.

Memory 82 may be configured to store a predetermined number of secondcardiac electrical segments, e.g., at least 1 and in some cases two ormore cardiac electrical signal segments, in circulating buffers suchthat the oldest segment is overwritten by the newest segment. However,previously stored segments may never be analyzed for noise detection andbe overwritten if an R-sense confirmation threshold is not reached atblock 714 as described below. In some examples, at least one segment ofthe second cardiac electrical signal may be stored and if not needed fordetecting noise (before noise analysis criteria are met), the segment isoverwritten by the next segment corresponding to the next R-wave sensedevent signal.

In addition to buffering a segment of the second cardiac electricalsignal, control circuit 80 responds to the R-wave sensed event signalproduced at block 706 by determining an RRI at block 710 ending with thecurrent R-wave sensed event signal and beginning with the most recentpreceding R-wave sensed event signal. The timing circuit 90 of controlcircuit 80 may pass the RRI timing information to the tachyarrhythmiadetection circuit 92 which adjusts tachyarrhythmia interval counters atblock 312. If the RRI is longer than a tachycardia detection interval(TDI), the tachyarrhythmia interval counters remain unchanged. If theRRI is shorter than the TDI but longer than a fibrillation detectioninterval (FDI), e.g., if the RRI is in a tachycardia detection intervalzone, a VT interval counter is increased at block 712. If the RRI isshorter than or equal to the FDI, a VF interval counter is increased atblock 712. In some examples, a combined VT/VF interval counter isincreased if the RRI is less than the TDI.

After updating the tachyarrhythmia interval counters at block 712,tachyarrhythmia detector 92 compares the counter values to an R-senseconfirmation threshold at block 714 to determine if noise analysiscriteria are met and to VT and VF detection thresholds at block 732 todetermine if a respective NID is met. If a VT or VF detection intervalcounter has reached an R-sense confirmation threshold, “yes” branch ofblock 714, the second cardiac electrical signal, e.g., from sensingchannel 85 is analyzed to detect noise corruption of the signal segmentthat may be causing false R-wave sensed event signals to be produced bythe first sensing channel 83, resulting in VT and/or VF counters beingincreased at block 712. The R-sense confirmation threshold may be a VTor VF interval count value that is greater than one or another higherthreshold count value. Different R-sense confirmation thresholds may beapplied to the VT interval counter and the VF interval counter. Forexample, the R-sense confirmation threshold may be a count of two on theVT interval counter and a count of three on the VF interval counter. Inother examples, the R-sense confirmation threshold is a higher number,for example five or higher, but may be less than the number of VT or VFintervals required to detect VT or VF. In addition or alternatively toapplying an R-sense confirmation threshold to the individual VT and VFcounters, an R-sense confirmation threshold may be applied to a combinedVT/VF interval counter. It is recognized that in some examples, VTdetection may not be enabled and VF detection may be enabled. In thiscase, only a VF interval counter is updated at block 712 in response toRRI determinations and the R-sense confirmation threshold may be appliedto the VF interval counter at block 714.

If the R-sense confirmation threshold is not reached by any of thetachyarrhythmia interval counters at block 714, the control circuit 80waits for the next R-wave sensed event signal at block 708 to buffer thenext segment of the second cardiac electrical signal. If the R-senseconfirmation threshold is reached at block 714, e.g., when the VFinterval counter is greater than 2, the control circuit 80 beginsanalysis of the second cardiac electrical signal segments for detectingnoise segments.

At block 716, control circuit 80 may retrieve one or more notch filteredsignal segments stored in memory 82. In some examples, the stored secondcardiac electrical signal segments are notch filtered and rectified bycontrol circuit 80 at block 716, e.g., by a software, firmware orhardware implemented notch filter and rectifier, after the R-senseconfirmation threshold is reached. In other examples, thenotch-filtered, rectified signal is received from the second sensingchannel 85 as shown in FIG. 4 and buffered in memory 82 for retrieval bycontrol circuit 80. As described above, the notch-filter may beimplemented to attenuate 50 Hz and 60 Hz line frequency noise.

At block 718, control circuit 80 determines a signal strength metric anda noise strength metric for use in identifying noise segments after theR-sense confirmation threshold is reached. The signal strength metricand the noise metric may be determined from each notch-filtered secondcardiac electrical signal segment buffered in response to an R-wavesensed event signal in some examples. In one example, control circuit 80may determine the signal strength metric as the maximum amplitude of thenotch-filtered rectified signal segment. Additionally, control circuit80 may determine the noise strength metric by determining a rectified,differential signal (or high pass filtered signal) and determining itsmaximum amplitude as the noise strength metric at block 718. The maximumamplitude of each buffered signal segment over a predetermined timeinterval, e.g., 1.2 seconds, and the maximum amplitude of the currentdifferential signal segment may be used by control circuit 80 todetermine if signal to noise criteria are met at block 720 in someexamples.

As described above in conjunction with FIG. 5 , the greatest maximumamplitude of all segments during the time interval may be compared to asignal strength threshold, e.g., one-third of the dynamic range of theADC or other selected threshold, and the differential signal maximumamplitude of the current cardiac electrical signal segment may becompared to a noise strength threshold, e.g., 13 ADC units or otherpredetermined value or selected threshold. When the greatest maximumamplitude is greater than the signal strength threshold and the maximumdifferential signal amplitude of the current signal segment is less thanthe noise strength threshold, the signal to noise criteria may be met atblock 720. In other examples, a ratio of the greatest maximum amplitudeover a predetermined time interval and the differential signal maximumamplitude of the current signal segment may be compared to a signal tonoise threshold ratio to determine if the signal to noise criteria aremet at block 720.

When the signal to noise criteria are met, the gain of the differentialsignal is increased at block 722 by control circuit 80. Otherwise, thegain remains unchanged. Control circuit 80 determines a noise metric,e.g., the noise pulse count, at block 724 using either the differentialsignal with unchanged gain or the differential signal with increasedgain. The methods for determining the noise metric by determining anoise pulse count as described above in conjunction with FIGS. 7-8 maybe performed at block 724. In other examples, a different noise metricsuch as any of the examples given above may be determined from thecurrent signal segment.

In addition to determining the noise metric, control circuit 80 mayoptionally determine the gross morphology metric(s) at block 726 fordetecting a tachyarrhythmia waveform morphology at block 726 asgenerally described in conjunction with FIGS. 9 and 10 above. Based onthe noise pulse count (or other noise metric), and the tachyarrhythmiawaveform morphology metric(s) when determined, the current secondcardiac electrical signal segment may be classified at block 728. Asdescribed above, when the noise metric meets a noise detection thresholdand the gross morphology metric(s) do not meet a tachyarrhythmiamorphology criteria, the segment may be classified as a noise segment bycontrol circuit 80 at block 728. When the noise metric is less than orequal to a noise detection threshold or at least one gross morphologymetric meets a tachyarrhythmia morphology criterion, the segment may beclassified as a non-noise segment by control circuit 80 at block 728.

After classifying the current segment, control circuit 80 may determineif the NID has been reached by the VT, VF or combined VT/VF intervalcounters at block 732. When a threshold number of intervals to detect(NID) is not reached by the VT interval counter, VF interval counter, orcombined VT/VF interval counter, control circuit 80 returns to block 710to continue determining RRIs and analyzing second cardiac electricalsignal segments as long as the R-sense confirmation threshold issatisfied (block 714). If the R-sense confirmation threshold is nolonger met at block 714, the noise segment counter or buffer may becleared at block 715.

When the NID is reached at block 732, based on the values of the VTand/or VF interval counters, control circuit 80 determines whether awithhold detection threshold number of noise segments has been reachedat block 734. The VT or VF detection based on the NID being reached iswithheld at block 734 in response to a withhold threshold number of themost recent cardiac electrical signal segments being classified as noisesegments. In one example, if at least two out of the most recent eightcardiac electrical signal segments are classified as noise segments, thewithhold detection threshold is met at block 734. The VT or VF detection(and any associated VT or VF therapy) is withheld by control circuit 80at block 736. The ventricular tachyarrhythmia is not detected by controlcircuit 80 when a threshold number of the most recent signal segmentsare classified as noise segments even when a tachyarrhythmia detectioncriterion, e.g., the NID, is reached. As long as the NID continues to bemet, control circuit 80 may continue to classify and count noisesegments as new R-waves are sensed to determine if the withholdthreshold is still being met at block 734.

In some examples, control circuit 80 may determine if terminationcriteria are met at block 738 when detection has been withheld.Termination of the fast rhythm may be detected based on a predeterminednumber of RRIs that are greater than a tachyarrhythmia detectioninterval or when a mean, median or other metric of RRIs determined overpredetermined time interval is greater than a tachyarrhythmia detectioninterval. For example, when a threshold number of RRIs longer than theVT detection interval (e.g., when VT detection is enabled) or longerthan the VF detection interval (e.g., when VT detection is not enabled)are detected subsequent to the NID being met, tachyarrhythmiatermination may be detected at block 738. In one example, termination isdetected at block 738 when at least eight consecutive long RRIs, e.g.,greater than the VT detection interval, are detected. In anotherexample, control circuit 80 may detect termination at block 738 when apredetermined time interval elapses and a median RRI is greater than theVT detection interval. For instance, when the median RRI of the mostrecent 12 RRIs is always greater than the VT detection interval for atleast 20 seconds, or other predetermined time period, control circuit 80may detect termination at block 738. Control circuit 80 may reset the VTand VF interval counters and the count of noise segments and return toblock 710 in response to detecting termination.

When the NID is met at block 732 and the withhold threshold is notreached at block 734, e.g., less than a threshold number of noisesegments being classified, a VT or VF episode is detected at block 740.Therapy delivery circuit 84 may deliver a VT or VF therapy at block 742in response to the VT/VF detection. It is to be understood that othercriteria besides the NID criterion may be applied before detecting theVT or VF at block 740. For example, various P-wave oversensing rejectioncriteria, T-wave oversensing rejection criteria, supraventriculartachycardia (SVT) rejection criteria, etc. may be required to be unmetand/or tachyarrhythmia onset criteria, tachyarrhythmia morphologycriteria, etc. may be required to be met before detecting VT/VF at block740 and delivering therapy at block 742.

It is contemplated that in other examples, the VT/VF detection may bemade in response to detection criteria being satisfied, e.g., the NIDbeing reached at block 732, but the VT/VF therapy may be withheld atblock 736 when the withhold threshold is reached or exceeded by thenumber of noise segments detected. Therapy delivery circuit 84 maywithhold a VT or VF therapy until the withhold threshold is no longerreached and the tachyarrhythmia is still being detected. Therapydelivery circuit 84 may deliver a withheld therapy when the withholdthreshold is no longer met and termination of the detected VT or VF hasnot been detected at block 738. If the detected VT or VF is determinedto be terminated at block 738 while the therapy is being withheld, thenoise segment count and VT/VF interval counters may be cleared and theprocess may return to block 710 without ever delivering a therapy.

It should be understood that, depending on the example, certain acts orevents of any of the methods described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of themethod). Moreover, in certain examples, acts or events may be performedconcurrently, e.g., through multi-threaded processing, interruptprocessing, or multiple processors, rather than sequentially. Inaddition, while certain aspects of this disclosure are described asbeing performed by a single circuit or unit for purposes of clarity, itshould be understood that the techniques of this disclosure may beperformed by a combination of units or circuits associated with, forexample, a medical device.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include computer-readablestorage media, which corresponds to a tangible medium such as datastorage media (e.g., RAM, ROM, EEPROM, flash memory, or any other mediumthat can be used to store desired program code in the form ofinstructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPLAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, a medical device has been presented in the foregoing descriptionwith reference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. A medical device comprising: a sensing circuit configured to sense at least one electrical signal and sense a plurality of electrophysiological events from the at least one electrical signal; a memory; and a control circuit coupled to the sensing circuit and the memory and configured to: store an electrical signal segment from the at least one electrical signal sensed by the sensing circuit in the memory in response to each one of the plurality of electrophysiological events sensed by the sensing circuit; determine that signal to noise criteria are met based on the stored electrical signal segments; determine an increased gain signal segment from one of the stored electrical signal segments in response to determining that the signal to noise criteria are met; determine a noise metric from the increased gain signal segment; determine that the noise metric meets noise detection criteria; and classify the one of the stored electrical signal segments associated with the increased gain signal segment as a noise segment in response to the noise metric meeting the noise detection criteria.
 2. The device of claim 1, wherein the control circuit is further configured to determine that the signal to noise criteria are met by one or more of: (a) determining a differential signal from at least one of the plurality of the stored electrical signal segments and determining that the differential signal meets the signal to noise criteria; (b) determining a maximum amplitude from each of a plurality of the stored electrical signal segments and determining that the maximum amplitudes meet the signal to noise criteria; (c) determining a maximum differential signal amplitude from at least one of the plurality of the stored electrical signal segments and determining that the maximum differential signal amplitude is less than a noise strength threshold; and (d) determining a maximum amplitude from each of the electrical signal segments that are stored within a predetermined time interval.
 3. The device of claim 1, wherein the control circuit is configured to determine the increased gain signal segment by: determining a differential signal segment of a most recent one of the electrical signal segments; and increasing the gain of the differential signal the most recent one of the electrical signal segments.
 4. The device of claim 1, wherein the control circuit is further configured to: determine a pulse amplitude threshold based on the increased gain signal segment; and determine the noise metric in response to the pulse amplitude threshold being greater than a suspected noise threshold amplitude.
 5. The device of claim 1, wherein the control circuit is configured to determine the noise metric by: identifying signal pulses of the increased gain signal segment; and determining a count of the identified signal pulses.
 6. The device of claim 5, wherein the control circuit is configured to identify the signal pulses by: determining a maximum amplitude of the increased gain signal segment; setting a pulse amplitude threshold based on the maximum amplitude; identifying consecutive pairs of zero crossings of the increased gain signal segment; determining a maximum pulse amplitude between each consecutive pair of zero crossings; determining a sample point number between each consecutive pair of the zero crossings; and identifying a signal pulse in response the maximum pulse amplitude being greater than the pulse amplitude threshold and the sample point number being greater than a predetermined width threshold.
 7. The device of claim 1, wherein: the sensing circuit is configured to sense the at least one electrical signal by sensing at least one cardiac electrical signal and sense the plurality of electrophysiological events by sensing a plurality of cardiac electrical events from the at least one cardiac electrical signal; and the control circuit is configured to store the electrical signal segment by storing a cardiac electrical signal segment from the at least one cardiac electrical signal in response to each one of the plurality of cardiac electrical events sensed by the sensing circuit.
 8. The device of claim 7, wherein the control circuit is further configured to: determine that a tachyarrhythmia detection criterion is met for detecting a tachyarrhythmia based on the at least one cardiac electrical signal; and withhold a tachyarrhythmia detection in response to the one of the stored cardiac electrical signal segments associated with the increased gain signal segment being classified as a noise segment.
 9. The device of claim 7, wherein the control circuit is further configured to: classify each of the stored cardiac electrical signal segments as one of a noise segment or a non-noise segment; determine that a threshold number of the stored cardiac electrical signal segments are classified as noise segments; and withhold a tachyarrhythmia detection in response to the threshold number of the stored cardiac electrical signal segments being classified as noise segments.
 10. The device of claim 7, further comprising a therapy delivery circuit configured to generate a tachyarrhythmia therapy in response to the control circuit detecting a tachyarrhythmia based on the at least one cardiac electrical signal; wherein the control circuit is further configured to: classify each of the stored cardiac electrical signal segments as one of a noise segment or a non-noise segment; determine that a threshold number of the stored cardiac electrical signal segments are classified as noise segments; and detect a tachyarrhythmia in response to tachyarrhythmia detection criteria being met by the at least one cardiac electrical signal; and the therapy delivery circuit is configured to withhold the tachyarrhythmia therapy in response to the threshold number of the stored cardiac electrical signal segments being classified as noise segments.
 11. The device of claim 7, wherein the control circuit is further configured to: determine a plurality of cardiac event intervals based on the sensed plurality of cardiac electrical events; determine that a first threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals; and store the cardiac electrical signal segments for determining that the signal to noise criteria are met in response to determining that the first threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals.
 12. The device of claim 11, wherein the control circuit is configured to determine that a tachyarrhythmia detection criterion is met by determining that a second threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals, the second threshold number being greater than the first threshold number.
 13. The device of claim 7, wherein the control circuit is further configured to: determine at least one gross morphology waveform metric from the one of the stored cardiac electrical signal segments associated with the increased gain signal segment; determine that the gross morphology waveform metric meets a tachyarrhythmia morphology criteria; and withhold classifying the one of the stored cardiac electrical signal segments associated with the increased gain signal segment as a noise segment in response to the gross morphology waveform metric meeting the tachyarrhythmia morphology criteria.
 14. The device of claim 1, wherein the control circuit is further configured to: control a therapy based on the electrophysiological events sensed by the sensing circuit; and ignore the electrophysiological event sensed by the sensing circuit that is associated with the one of the stored electrical signal segments classified as the noise segment.
 15. A method comprising: sensing at least one electrical signal; sensing a plurality of electrophysiological events from the at least one electrical signal; storing an electrical signal segment from the at least one electrical signal in response to each one of the plurality of sensed electrophysiological events; determining that signal to noise criteria are met based on the stored electrical signal segments; determining an increased gain signal segment from one of the stored electrical signal segments in response to determining that the signal to noise criteria are met; determining a noise metric from the increased gain signal segment; determining that the noise metric meets noise detection criteria; and classifying the one of the stored electrical signal segments associated with the increased gain signal segment as a noise segment in response to the noise metric meeting the noise detection criteria.
 16. The method of claim 15, wherein determining that the signal to noise criteria are met comprises: determining a differential signal from at least one of a plurality of the stored electrical signal segments; and determining that the differential signal meets the signal to noise criteria.
 17. The method of claim 15, wherein determining that the signal to noise criteria are met comprises: determining a maximum amplitude from each of a plurality of the stored electrical signal segments; and determining that the maximum amplitudes meet the signal to noise criteria.
 18. The method of claim 17, wherein the control circuit is configured to determine that the signal to noise criteria are met by: determining a greatest maximum amplitude of the maximum amplitudes determined from each of the plurality of the stored electrical signal segments; determining that the greatest maximum amplitude is greater than a signal strength threshold.
 19. The method of claim 15, wherein determining that the signal to noise criteria are met comprises: determining a maximum differential signal amplitude from at least one of the plurality of the stored electrical signal segments; and determining that the maximum differential signal amplitude is less than a noise strength threshold.
 20. The method of claim 15, wherein determining that the signal to noise criteria are met comprises determining a maximum amplitude from each of the electrical signal segments that are stored within a predetermined time interval.
 21. The method of claim 15, wherein determining the increased gain signal segment comprises: determining a differential signal segment of a most recent one of the electrical signal segments; and increasing the gain of the differential signal the most recent one of the electrical signal segments.
 22. The method of claim 15, further comprising: determining a pulse amplitude threshold based on the increased gain signal segment; and determining the noise metric in response to the pulse amplitude threshold being greater than a suspected noise threshold amplitude.
 23. The method of claim 15, wherein determining the noise metric comprises: identifying signal pulses of the increased gain signal segment; and determining a count of the identified signal pulses.
 24. The method of claim 23, wherein identifying the signal pulses comprises: determining a maximum amplitude of the increased gain signal segment; setting a pulse amplitude threshold based on the maximum amplitude; identifying consecutive pairs of zero crossings of the increased gain signal segment; determining a maximum pulse amplitude between each consecutive pair of zero crossings; determining a sample point number between each consecutive pair of the zero crossings; and identifying a signal pulse in response the maximum pulse amplitude being greater than the pulse amplitude threshold and the sample point number being greater than a predetermined width threshold.
 25. The method of claim 15, wherein: sensing the at least one electrical signal comprises sensing at least one cardiac electrical signal; sensing the plurality of electrophysiological events comprises sensing a plurality of cardiac electrical events from the at least one cardiac electrical signal; and storing the electrical signal segment comprises storing a cardiac electrical signal segment from the at least one cardiac electrical signal in response to each one of the plurality of cardiac electrical events sensed by the sensing circuit.
 26. The method of claim 25, further comprising: determining that a tachyarrhythmia detection criterion is met for detecting a tachyarrhythmia based on the at least one cardiac electrical signal; and withholding a tachyarrhythmia detection in response to the one of the stored cardiac electrical signal segments associated with the increased gain signal segment being classified as a noise segment.
 27. The method of claim 25, further comprising: classifying each of the stored cardiac electrical signal segments as one of a noise segment or a non-noise segment; determining that a threshold number of the stored electrical signal segments are classified as noise segments; and withholding a tachyarrhythmia detection in response to the threshold number of the stored electrical signal segments being classified as noise segments.
 28. The method of claim 25, further comprising: classifying each of the stored cardiac electrical signal segments as one of a noise segment or a non-noise segment; determining that a threshold number of the stored cardiac electrical signal segments are classified as noise segments; detecting a tachyarrhythmia in response to tachyarrhythmia detection criteria being met by the at least one cardiac electrical signal; and withholding a tachyarrhythmia therapy in response to the threshold number of the stored cardiac electrical signal segments being classified as noise segments.
 29. The method of claim 15, further comprising: determining a plurality of cardiac event intervals based on the sensed plurality of cardiac electrical events; determining that a first threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals; and storing the cardiac electrical signal segments for determining that the signal to noise criteria are met in response to determining that the first threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals.
 30. The method of claim 29, further comprising determining that a tachyarrhythmia detection criterion is met by determining that a second threshold number of the plurality of cardiac event intervals are tachyarrhythmia intervals, the second threshold number being greater than the first threshold number.
 31. The method of claim 15, further comprising: determining at least one gross morphology waveform metric from the one of the stored cardiac electrical signal segments associated with the increased gain signal segment; determining that the gross morphology waveform metric meets a tachyarrhythmia morphology criteria; and withholding classifying the one of the stored cardiac electrical signal segments associated with the increased gain signal segment as a noise segment in response to the gross morphology waveform metric meeting the tachyarrhythmia morphology criteria.
 32. The method of claim 15, further comprising: controlling a therapy based on the electrophysiological events sensed by the sensing circuit; and ignoring the electrophysiological event sensed by the sensing circuit that is associated with the one of the stored electrical signal segments classified as the noise segment.
 33. A non-transitory computer-readable medium storing a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to: sense at least one electrical signal; sense a plurality of electrophysiological events from the at least one electrical signal; store an electrical signal segment from the at least one electrical signal in response to each one of the plurality of sensed electrophysiological events; determining that signal to noise criteria are met based on the stored electrical signal segments; determining an increased gain signal segment from one of the stored electrical signal segments in response to determining that the signal to noise criteria are met; determining a noise metric from the increased gain signal segment; determining that the noise metric meets noise detection criteria; and classifying the one of the stored electrical signal segments associated with the increased gain signal segment as a noise segment in response to the noise metric meeting the noise detection criteria. 